Ohhhh boy, I could make a whole damn essay about dropped clues in Errant... There are still things in the prologue that haven't paid off yet that only one or two people have noticed, and one that nobody has... 
I've collapsed these into categories because SO MANY SPOILERS and there are a lot!:
Prologue
Page 3 - Rekki having a sister who Sarin thinks might be the reason she's so desensitised to spirits is noteworthy.... And won't pay off for over a hundred pages when Rekki's sister starts turning up and is revealed to be a Changeling; a powerful fey in a human body.
Page 5 - This one is just really funny and nobody picked up on it. There's a panel where Sarin facepalms:
Then much later, after the timeskip, in chapter 3 the mysterious Crow... Does a really similar reaction to people's bad decisions:
This was 100% a deliberate clue and I was kind of sad people missed it. 
Page 7 - Hey, it's Jules! Lil' baby ten year old Jules! It's explicitly mentioned by Sylvia that Urien is Jules' brother here, which by the time it became plot critical, a lot of people had forgotten, so I could play it like it's a reveal in chapter 3 (because to Subo, it is), but it's actually just a reminder.
They already call Sarin "The pretty one". Which foreshadows their later reaction to grown-up Sarin:
Also really worth noting here how deadpan this young Jules is, contrasting the very theatrical entrance they'll make after the timeskip. It was an intentional clue that Jules is putting on a persona based on theatrical villain characters... but that wasn't explicitly spelled out until waaaay over in chapter 5. Young Jules' deadpan face is also maybe your very first clue this kid is autistic.
Page 9 - A young Sylvia, perhaps somewhat Romanticised by Sarin's imagination, returns Excalibur and ends the Third Age of Knights, cradling Athena Lune. This is really important for various reasons that haven't paid off yet.
Page 16-17 - Rekki flares her aura here.... and her hair turns a little redder and she pulls off a...rather superhuman attack. This is the first sign of Rekki's ability to somehow absorb magic externally, rather than producing magic with her body like the other characters.
Also, "The strongest magic is the power of friendship!" and Urien shooting it down, is played as a joke, but it's pretty thematic. Pretty much every problem post-timeskip has required the knights to work as a team to overcome it.
Page 26 - You might be thinking "It really doesn't seem like Urien has a plan here." And yes, it certainly does seem like maybe he's not a mastermind at all, just a brazen opportunist, doesn't it? (Goddamn Glass Onion coming out before I could pay this off and doing the same twist...).
Page 27 - Exposure to the Eye of Brilliance opening has massively boosted Sarin's magic count and now her hair is violently blue. Her aura effect on subsequent pages is very powerful, but she didn't even use her aura earlier. As an adult waaay over in chapter 6, we'll see that her aura is so powerful, she can slice imps in half without even needing her sword. This is also why certain...changes later are at first misidentified as "normal magical mutation".
Page 29 - Ambrosius bleeds. None of the other demons in subsequent chapters are shown getting cuts or bleeding liquid blood like this... until we come across a cambion in chapter 5. Other early clues that he was a Cambion were his comparatively scrawny build rather than the cartoonish beefiness of most demons, and his dirty robe contrasting the more "costumed" look of demons that don't have a flesh and blood body, and him being pretty smart and having motivations other than "FIGHT ME!"
So that makes the fact that he appears to poof like a standard demon on page 34 weird... doesn't it? The explosion seems a little brighter and fancier than usual somehow... Ah, I'm sure it's nothing....
Oh also on those pages, Sarin's aura! It shoots blue projectiles that go "shing!" and cut things. Which we see Crow do later. Oh wow, it sure is weird that Rekki doesn't seem to recognise such an obvious clue, huh?
In between: The main series cover! Sarin is sitting down in this compostition because if she was standing up next to Rekki, you'd see the height difference very clearly. Also there's a clue by her foot.
If you guessed from the sword with very similar characteristics, like a rectangular blade and a hole in the pommel, that Rocket was the sister of Rekki's mentioned in the prologue, good job! People rarely notice though that Rekki and Rocket actually have the same nose, eye shape and skintone. Urien and Jules similarly have the same distinct eyes and nose (and weird anime hairline hahahaha!) and skintone.
Chapter 1
Page 2 - There's a lady with green hair handing her notice to Felix here and saying "sorry". Foreshadowing that in a few pages, he's going to run into the lounge looking for a new support knight for Rekki, and then a few pages after that, we'll find out Rekki thought her support knight had gone to get coffee.
Page 15 - There's Crow graffiti on the wall here, and Crow's "NO KINGS" slogan. There was another "NO KINGS" on the first page of this chapter. This pays off later in the chapter when Crow becomes a key character.
Page 28 - Yeah, Urien already has a pretty strong suspicion of Crow's identity here. Specifically pointing out that the speech "reeks of Aoki". Also the Crow panels and this whole scene is tinted blue because I'm smacking you around the head with this that hard.
Chapter 2
Page 6 - Oh hey, it's Jules! Jules has a sensor network all over the city, you say? Yep, that pays off waaay over in chapters 5 and 6 where we discover that Jules is kind of in charge of Joysgard when Rekki's not around, and also that their sensors at the University keep breaking. Another small detail: Rekki's phone pic of Jules is very natural and awkward, not like Jules' villainous persona at all. A hint that Rekki knows the "real Jules".
Page 11 - Rekki threatens to call The Wanderer Without the Forest, and the fey and ghosts immediately comply. Who's Wanderer Without the Forest? Well, that pays off over in Chapter 5! It's Rekki's sister, Rocket, who is also "First among Queen Nimue's Hobgoblins".
Page 16 - Jules calls themselves "The Princess of Darkness" here...and a lot of people didn't pick up on the fact, at the time, that this isn't completely a joke. As established when we saw little Jules in the prologue, Jules is Urien's sibling, and so since he's King.... Yep! Jules is a Princess, and their position as heir to Excalibur is actually really plot important.
Page 24 - MASSIVE overlooked detail that just became relevant literally in the most recent update of the comic: Rekki tells Jules "You're the key to fixing this whole mess; you have to stay safe."
YEP. Rekki was already on team "Jules needs to get Excalibur off Urien" and was ready to die for it.
Page 28 - Some people caught this one at the time, but Rekki has no phone history with Sarin here, and the picture of Sarin in her contacts doesn't look like adult Sarin. It's a pretty strong clue of just how long the two haven't been communicating (later turns out to have been 6 years since they last spoke.) If you've read ahead and know how gaunt and scary she starts looking later, you can probably guess this photo is probably Sarin aged 15.
Page 31 - You might guess, from how annoyed they seem, that Jules is on the phone to Urien. That's why he turns up later.
Page 35 - In spite of later seeming weirdly to not buy into the theory that Crow is Sarin later... Rekki's immediate gut instinct is absolutely on the money:
Chapter 3
Oh boy welcome to: Crow's Identity is Foreshadowed SUPER-HARD....
Page 1 - The big demon calls Crow a Tengu. This'll be important later.
Also, a lot of people thought Crow wasn't Sarin due to the height difference between her and Rekki after them having been pretty close in height at age 14. Now you know why Sarin was sat down on the cover... to keep that clue hidden longer.
Page 8 - When Crow is tired, their magic sword goes smaller and more katana-shaped. Then on page 15, Crow even invokes the name of Sarin's favoured deity, Sulis Minerva, which is practically Sarin's catch-phrase (a lot of people caught that one), and then page 16, the sword goes full-on obviously a large katana.
Page 21-34 - This whole sequence is really funny to go back and read because Crow clearly doesn't recognise Jules, and isn't being nearly as guarded with their identity as they would if they know they were talking to Urien's sibling. Casually revealing their MSc in Arcane Science, dropping out of character completely with a much gentler way of speaking and feminine body language, including some very typically Japanese gestures, like pointing at their nose for "me" and indicating the other person with an open hand. This pays off in just how ANGRY Sarin is on finding out who Jules is in chapter 4; she's angry with herself.
Page 34 - "I won't gossip about what's under your clothes". Jules holds 100% true to this promise. At no point in subsequent chapters does Jules let anything slip about knowing who Crow is. But seeing how Jules interacts with Sarin in chapter 6.... it's pretty clear they know they're talking to the same person as the previous day, even dropping hints like "It's good to see your face" rather than "see you".
Another thing of note on this page: Jules' secret aura effect! This will be important later!
Chapter 4
Page 1 - Small detail: trophy holding up a sword on Rekki's shelves. In chapter 5 we find out that Rekki used to do competitive swordfighting or "Melee" (a Medieval knightly tournament sport) in school, and that's actually how she met Sarin.
More important detail which still somehow went over people's heads... Rocket lives in Rekki's house and on her shelves has pictures of herself with Rekki and their parents, as well as one of her with Rekki and Jules (I can't believe at this point some people hadn't twigged that she's definitely the sister mentioned in the prologue).
Page 9 - Sarin was introduced sitting down, but she stands up here. Nobody picked up on it, but she is pretty tall! If somebody had gone back and checked, they'd have seen that Zeph is taller than Jules, who is taller than Rekki, indicating that Sarin is significantly taller than Rekki.
Page 16 - Speaking of Sarin and sitting, here's her chair at the round table. Later, in chapter 5, we discover she's never actually sat in it. This is a reference to the siege perilous of Sir Galahad from Arthurian mythology.
Page 20 - Rekki almost loses her temper with Urien here... but notice how when she chooses to back down and apologise the panel is purple? That's relevent.
Page 28 - Rekki has now been told by three different people, Crow, Urien AND Sylvia to talk to Sarin, and they all have different things they want her to do in collaboration with her. This leads to this really funny page in Chapter 6:
And that's where I'll leave things, because a lot of chapter 5 and 6 has been paying off all these little things that were set up. I always enjoy when people go "I just re-read the early chapters and OMG!" 
