One that comes to mind for me is the appearance of an annoying familiar animal when you don't expect one. This happens, for instance, in the fantasy You Slay Me - the main character summons a minor demon which incarnates as a smelly mangy dog that likes making gross jokes and licking itself. Ugh.
I put down the mystery On What Grounds yesterday because of another plot device - the main character has been tricked into sharing an apartment with her ex-husband, with whom she is not on good terms. I couldn't really cope with the idea of the two of them sniping at each other throughout the book, unable to get away.
How about you?