Ok, when I say oh no moments I mean like that moment when someone points out such a deep plot hole you never knew was there and it's so bad you have to almost rewrite over 10, 000 words. To where you might as well put happy jazz music over your agony. I ask because I've had them too and wish to know that I am not alone in this.
My moments tended to be more of a head thumping on the keyboard muttering oh f***..
You're not along. Surprisingly enough though... sometimes you can fix it with just a line or two. Recently did that.
I have never had a reader point out the plot hole but I luckily realized one myself in my first story. I noticed it very early on but it pretty much affected the way I had to approach the story and how I built the tension between the characters.
A "brief" explanation for it: The main character Em has been bitten by a werewolf and then turns into one. She runs to the woods and her parents come looking for her. She leaves them alone when she's called by another wolf (the one who bit her).
And what I had planned was for the other wolf to tell that Em had almost attacked her parents so she would go with him. And then it would be revealed later on that he had lied, and this would cause some drama between them.
But I realised that there was no logical way he would know that since he wasn't present in the situation, so he didn't even know Em had met her parents since she didn't remember it either.
Hey, having people pointing out plot holes is great. Especially in an earlier stage!
Also how I deal with it is bullshitting my way. Recently I discovered something that might rise some questions, but I try to bullshit my way and make the situation more vague.
Not really a plot hole, but a bit of an "oh no."
Early into my run of posting my comic on Tapas, I got a review from @darthmongoose about my webcomic Hollow, which was mostly very positive, but she mentioned (and I'm paraphrasing here), "Man, I hope Levi's whole thing isn't just that he's in love with Skyler and they kiss and make up." And also, "Levi seems all cool swooping in with his magic coat and saving the day, but it's only because he was right. If he was just wandering around in a magic coat for the last six years with no magic around, then he looks like a doof."
And I was like...
Because that's exactly what I had written in my original draft of the comic. And she was entirely right in pointing those things out, they were basic character and story flaws that I hadn't noticed myself. Levi needed better personal motivations, and I needed to balance out Skyler's perspective of "the magic is gone forever, I don't even carry the Amulets around anymore, better get a retail job and go to college" with Levi's perspective of "I still keep a spellproof coat in the trunk of my car when I go grocery shopping because the magic could come back at any moment, I can't move on with my life in any other way."
As a result, I rewrote a huge section of the story, which made it so much better, made Levi's character a lot more nuanced and with actual character depth and motivations beyond just "I have a crush on a guy I haven't seen for six years" (though he still kind of thinks that's his motivation), and honestly kind of informed an entire new moral meaning to the story. It helped literally every aspect of the story in ways I wouldn't have considered myself, because I was leaning a bit too hard on the nostalgia tropes and not leaning hard enough on the "reality" side of the tropes, which is the whole point of my story.
So, it was an Oh No. But it turned out to be something really amazing for my story, and I'm very glad it happened.
My big 'oh no' moment was when I saw Covid was starting to become a big thing, and my comic main plot is based on a epidemic disease 
I had to rewrite a big chunk. I'm still upset about it, but feel ok with my story now.
Mine came about a week ago when, after having now read far more broadly and really developed a better sense of the webcomic medium and what I like in it, I realised that the pacing in my early episodes is nowhere close to my ideal. (It moves too fast. I need to slow it down.) And also, that I missed a prime opportunity to establish how cluttered and untidy my protagonists' ship is in those early panels.
It's okay, you live and you learn. It's my first comic/webcomic, so clunky things were bound to happen. If I can pull my buffer far enough ahead in the coming months, I'll re-do those early pages/episodes later this year. But ugh, realising it was a bummer.
The biggest "oh no!" in my first draft of "Red Shift" was when one of my beta readers pointed out I kept flipping from third person limited (Steppe's point of view, you only read what his mind knows) and third person omniscient. I burst into laughter upon being told this and went to work fixing what I had written up to that point. (6 chapters only, thankfully!) Yes, Steppe someone how knows all the secrets and thoughts of the Royal House even though none of them will really speak with him except his sister (the Queen). 
I take the Bob Ross method and turn my mistakes into "happy little trees". haha. With my current series, I haven't had any big "OH NO" moments but I have had a lot of loose plot threads that I've had to keep track of and weave them back in.
Somehow I’ve caught my own plot holes (since I constantly re-read my work) before any of my readers have. But that makes for some stressful chapter writing to try to clear it up before it gets caught.
But I think if a reader pointed one out I’d fill it, whether in future chapters or past chapters if I had to. And then you can always send the update that sends to readers with a note if you wanted. Or you can just do it silently and fix it for future readers. We’re only human!
I will continue to keep re-reading as I go. I’ve not only caught plot holes or errors, but I’ve created more plot lines based off of or tied into things I’d forgotten I’d written about.
Mine came about 4 years ago when my readership dropped like a lead balloon and realised that I was way worse at writing and art than I initially suspected.
I've been on this uphill battle ever since trying to improve and to keep whatever readers I have left.
It's not going too well.
Me: Introduces absolutely horrible piece of shit character based of an individual in my life that's permanently traumatized me and is absolutely nothing but vile and horrible to the main character
Readers: I ship them!!!! 
My first "OH NO!" Moment was when readers hated my prologue's huge chunk of text. But that's extremely important to the story!!!
I redrew it, made it more "show" instead of "tell" but it made me realize that most like nobody will stay around until the end for it to payoff. So it will be a waste of first episode for many readers. 🥲 
My biggest oh no is reading the news about corrupt figures in politics/tech/finance and realizing my villains probably aren't evil enough.
I'm still working on that one.
My 'oh no' moment isn't really a plothole... it's the fact that I accidentally over-commit to a series that I planned as casual and sporadic...... and it's a series that I haven't finished the first draft yet, so I'm in constant chase with my own impulsively-decided twice-a-week updates and the backlogs. Terrible decision. I forgot I always feel bad when I make people wait.
Idk if considered an "oh no" moment but I feel it is...
Somehow this one reader reading into the main girl character too much and assuming about her on the fly? The reader assumed the girl was making "everyone's" problems about herself. I had to reread the previous chapters to check if this was true. Kinda wish I had more reviewers to see if that was the case.
I've thankful not had anyone point out any plot holes (yet) but I've had a few comments point out minor issues with continuity or grammar or whatever, stuff that can be fixed in a few minutes 
Really grateful for that kind of feedback though, because it means I can improve on my work :3
Mine is that I have no fucking idea how to make the sci fi convincing and that I have no idea how to further the plot other than a very vague idea. Also no idea of how to go about the next chapter but uhh such is life.
I'm fighting my instinct to apologise profusely for the inconvenience to say I'm very happy it was helpful and turned out well in the end! 
I have another Oh No moment related to darthmongoose as well : P I had relatives say they didn't get the start of the comic much back when it was just half Chapter 1, and I just assumed it was unfamiliarity with the fantasy genre. Nope, get to that review, and I really am throwing terms out out of my own experience with the genre and not taking into account the reader might not have read exactly what I have.
I think it enhanced the comic majorly to hit that hard obstacle - as it turns out later, when I got to exposition, I was also cluttering up the dialogue("I have heard this spell was used in this form" rather than "Someone set fire to this") and making text boxes massive as a result, and rewriting them to just use simple terms rather than magic stuff helps out immensely.
