Clannad - I got about 6 episodes into the first season and was already sick of all the characters. Nobody in the show was particularly likeable and the MC’s best friend character was particularly obnoxious. I have since had people tell me that I just need to bear with it and that the real story hits in After Story, but I just couldn’t.
Science Fell in Love, So I Tried to Prove It - This show was actually pretty decent and I was really enjoying it. I wouldn’t say it was the best romcom by any stretch, but it was competently executed with an original cast of characters. The math/science elements were also really well done as was capturing the feel of graduate school; a setting that you rarely see portrayed in anime. That is something I am sensitive to since I am a scientist with a PhD, so I am sure that added to my enjoyment. However, the final ~2 episodes were so terrible, it pretty much poisoned the rest of the show. I don’t want get into spoilers, but I have yet to see so much good be undone by an ending in any other show.
Two spring to my mind:
You save yourself some heartache with Clannad and not watching After Story.