Near the end as the group begins singing, Pierce is seen throwing away his present to Troy, then after Jeff enters again we see him somehow throw it away again.
In one of the alternate timelines, Annie's purse falls to the floor and her revolver discharges. As the pistol appears to be a Smith & Wesson Chief's Special, such a discharge is absolutely impossible.
Abed's rolling boulder model clearly says "Adios, Sapito." However, the movie character was named Satipo, not Sapito.