I think my only real "oh no" so far was what happened to my first comic's non-existent "Season 2" 
I wrote my first project here to be a short-ish one shot story, but left it with an open-ended cliffhanger(ish?) ending that could either act as a suitable ending point, or lead into a follow up story if I felt like it. The comic took about 21 months to finish, from initial writing to posting the final page on tapas.
Around month 18 or so I felt like the comic was doing really well and I was having a lot of fun with it, so I started hyping myself up to pursue the continuation option after all and create a "Season 2" for the story! I started casually talking about it here on the forums, in replies to comments on my series, in some of the final episode author notes, and even ended the story with a big "End: Season 1" at the bottom of the page as if there was more to come.
Following the ending of my comic I took a bit of a break, I had a large commission project come up that ate a couple of months, spent another month or so after that just chilling, then I sat down to start writing Season 2. I had begun outlining it months before, but I hadn't gotten down to the nitty gritty details or anything yet. And... I quickly realized that I didn't like where it was going and didn't think that it warranted another 1-2 years of my time
The issues were multi-faceted: from the hasty world building decisions that were made to service the specific story from "Season 1" but didn't work as well as I had hoped for a larger world, to the increasingly hard to work with insert characters (the MC was a loose self-insert, and the rest of the main party from season 1 were friend-inserts, season 2 was only going to retain the MC but still), and just realizing that the series didn't really have the long-term potential that I kinda thought it did a few months before.
So I had to kind of haphazardly go leave comments and author's notes on tapas and webtoons explaining that Season 2 wouldn't be happening anymore and to "expect news for a new comic soon!" like a year ago... and my new comic just launched earlier this month lmao. Certainly not the biggest "oh no", but it still wasn't a fun time xD Especially since a lot of my most invested readers had been quite looking forward to "Season 2" since I had been hyping it up for months and then... I just didn't deliver anything beyond some inktober art of the new cast in 2019 x_x