Michael Fassbender in X-Men First Class, where he keeps randomly going Irish. I think it was claimed that he did it deliberately, in tribute to Sean Connery. I don’t know what accent he was actually going for, I wish he would have just picked one though.
Kevin Bacon’s German accent when speaking German, also in XM: FC. Now, I only have GCSE German, but it sounded pretty bad!
Julia Landau in Buffy/Angel.
Keanu Reeves and Winona Ryder in Bram Stoker’s Dracula. DUUUDE, IT’S TOTALLY A VAMPIRE!
Peter Dinklage in Game of Thrones, but he gets a pass for being awesome, and because he’s depicting a character from a fictional land, so therefore can’t actually get the accent wrong.