Harry Potter and the Mistaken Prince
If you're like me (a nerd), you probably love catching movie mistakes in films. MovieMistakes.com is a great site to see a list of all known mistakes, and their Half-Blood Prince page now holds 66 of 'em! Like this one:
When Hagrid and Slughorn are drinking after Aragog's funeral, Slughorn's cup is considerably larger than Hagrid's cup. After Hagrid passes out, Slughorn's cup has shrunk to the same size as Hagrid's.
I was surprised to see that they didn't have one I caught: after Draco is escorted out of Slughorn's party by Snape and Harry begins to spy on them, you can clearly see crew rugs used to quiet Draco and Snape's footsteps. The same rugs are visible earlier in the film when Harry and Ron walk down a corridor after Slughorn's class.
(I just sent that to MovieMistakes.com.)