I don't mean completely acceptable reasons like poor hygiene, lying, flakiness etc. I mean things that the poor other person could not possibly have anticipated would mean the end of their relationship.
For me it was many years ago when I finally finally got together with a lad I'd had a crush on for about 3 years. About 2 months after we started seeing each other he invited me to his house. Which is where he committed the unforgivable sin of picking up a broom and sweeping the kitchen floor.
That was it. 3 years of obsessing over him and, just like that, a bloody sweeping brush came between us.