When filming a movie the actual filming of scenes takes place all throughout the day and sometimes on different days. Because of that, things on the set and the actors themselves tend to change a bit and thats how you get movie screw-ups
If you go to the Internet Movie Database you can look for any movie and find the goofs/fuck-ups in it
http://www.imdb.com
Here's one from the Hangover
Crew or equipment visible: During the scene with Mike Tyson, you can see the reflection of the crew and equipment in Phil's sunglasses.