When Piper wants to clean up the potato chips, the bowl is completely empty. Suddenly someone appears and you can see there are already potato chips in the bowl before Piper starts refilling the bowl.
At the 16:09 mark the SGT is talking to the boys and the shot has his patches but the 82nd AB patch is on upside down. The AB guys must love that.
When Phoebe catches the blue balloon and follows Piper down the hallway, all the shots from behind were clearly filmed using a stand-in and not Alyssa Milano. The woman in question has paler skin and is significantly taller.