Call it the "Knocked Up" syndrome -- when an attractive actress is romantically paired onscreen with a guy she'd never give a second thought to -- but Hollywood has made quite a habit of casting unrealistic and unbelievable romantic pairs. Even before Seth Rogen made a baby with Katherine Heigl, stars like Woody Allen and Chevy Chase were casting themselves opposite knockout leading ladies. (Chase famously admitted that he wanted Beverly D'Angelo cast as his wife in the "National Lampoon"'s series because she was so pretty.) And guys aren't the only ones guilty of this (cough, Queen Latifah, cough).
Check out our list of wildly mismatched onscreen couples we just can't believe.