Character mistake: After the dodgeball game ends Jerry asks the referee if skyzone had a bar, but in the background he's already walking away.
Continuity mistake: Grandpa squeezed the mustard bottle, aiming at and hitting the center of Peter's pants where the zipper or "fly" is, plus some mustard landed on his shirt. The stain starts off covering a pretty large area, then shrinks to half the size and moves further right. (01:13:59 - 01:22:59)
Revealing mistake: The boards around the window on the house that got hit with a falling tree fell off separately - individual boards were not connected enough to stay attached. As each board fell, the "wall" around the window could be seen and it was so thin (no thick frame) that it was obvious the house (or that section of the house) was a prop. The roof also seemed far too damaged to be "real" - there was a huge hole in the roof but the falling tree did not have long enough branches to cause that large of a hole. (01:17:20)
Continuity mistake: Grandpa did not get a lot of soda/pop sprayed on his shirt or Santa hat - most went onto his face. However, the "wet area" of his shirt becomes much larger after he left the table. Not too long afterwards, the "wet spot" on his shirt looked much wetter, then soon after he falls down holding onto the ladder it's drier. Also, the white fur around his Santa hat stayed almost dry (or dry) despite being on his head when the soda/ pop (can shaken by Peter) sprayed all over Grandpa's face. (01:15:30 - 01:15:53)