This is something that happens in a lot of films, planes in the sky in Jane Austen films, music playing in the background that wasn't released when the film was set etc.
I discovered some really obvious ones in some big films:
The Others http://www.youtube.com/watch?v=XRfNxcETO5g
American Psycho http://www.youtube.com/watch?v=_ZLDJwEfPGE
There Will Be Blood http://www.youtube.com/watch?v=a03NXSH1TZs
Pleasantville http://www.youtube.com/watch?v=5KDuIFfqUSQ