Shallow reasons (different people):
She did not actively participate during sex.
She talked about Dan Brown as though he was a historian.
She did not really look like her profile pic...not horribly ugly or anything, just not as expected.
The first date included her being very excited about Twilight and how she had gone to Forks when she read the first book.

None of these were really a "stop dating" thing, as we never really started dating. But shallow reasons appear early.

He always insisted on 3 lemon slices when ordering tea.

Granted i only dated this guy because he was trying sooooo hard so i eventually gave in...he acted very effeminate especially after a few drinks. i'm quite the tom boy so im hard core unattracted to someone less manly than me. He was older, (i pretty much only date older so we are more mentally equal) but not too bright. He also had split custody of an awesome little boy but on days we hung out all together he would do weird shit like, to act like i was his mom or something... like whisper "hey can you tie his shoe?" Then announce, aw see she didnt want a little buddy to fall! And tell him he had to hold both our hands when going through parkinglots and such. It was very jarring as a 22 or 23 year old. I helped my parents raise my nephew til he was like 5 so thats not new but to straight up be thrown into being step mom was uncomfortable.
But i always wondered if i was just over thinking it and being a shallow asshole....