Harrison Ford in K-19.
Halle Berry's "what is that even trying to be" accent in X-Men, which she just completely abandons in the next movies, just as well. For that matter, wtf kind of accent is Paul Bettany doing in The Da Vinci Code? He sounds like Dracula.
Sam Worthington in Terminator Salvation, can't even maintain an American accent for two sentences.
Ditto for Marion Cotillard in Public Enemies, who has at least three scenes where she sounds totally French.
I may piss some off here, but Alan Rickman's AND Jeremy Irons' one scene attempts at American accents were horrible.
Love Michael Caine, but his American accent in The Cider House Rules wanders back to, well, Michael Caine after about three sentences.
Val Kilmer starts off in The Ghost and the Darkness with a half-assed Irish accent, then completely forgets about it for two-thirds of the movie, and then toward the end out of nowhere he randomly pronounces "schedule" the British way. Kind of funny. Ironic too, because in The Saint, Kilmer does like 100 different accents. Not sure how well he nails all of them, but he puts a hell of a lot more effort in there, at least.
Ralph Fiennes and Clive Owen can't do American accents to save their lives.