One of my favorites is when the girl rides the guy soooooo good that he can't take it, and his cock pulls out too late to stop the inevitable... even better is if he's "pinned" by her being on top of him so he can't shoot the kind of cumshot he was planning... the more uncontrolled the better... the best would be hands-free of course, or maybe even better would be if she sees what's happening and tries to do the job herself with her hand while he flails around underneath her, pissed off at himself for cumming early.