Lying about events that allegedly happened while we were engaged, and expecting me to actually believe her. Specially when she switched who did what.
For example. "I even cooked at late night for you!" - err... no. She only cooked for me a handful of times, all of them afternoon snacks. (I'm not a big fan of afternoon eating but I appreciated the gesture.) I was cooking for her fairly often, including preparing a "fake" barreado (kind of stew) at 3AM.
Or claiming that a common friend (a woman) openly mocked her while we were drinking with friends, and I did nothing. I was there and I remember what friend said - a single sentence, roughly "hello and bye for both of you!", since that friend was leaving as we were arriving on the bar. Even if she wanted to mock my then girlfriend, she wouldn't have the time to do so.
What makes those funny is not the fact that she was lying, but that they're such idiotic lies that you can smell the bullshit from a kilometre of distance.