When Jay Baruchel and Craig Robinson are running from the dog-like demon in the neighbor's house, Craig saves Jay by pushing him out of a glass door. As Craig is jumping into Jay he is securely wearing the backpack full of food but when they crash through to the outside the very next second he is not wearing it.
The goof items below may give away important plot points.
When Jay Baruchel and Seth Rogen make it to Heaven and see Craig Robinson, Jay starts yelling, "Yay!" as both Jay and Seth run to Craig. Jay yells the whole time until they reach Craig, but when they reach him the audio for Jay is still going while his mouth is closed.
When they first break through the floor/ceiling, there is no rebar in the concrete (which they would not have been able to break with their improvised tools anyway.) When the hole is next shown a few sequences later, some severed stumps of rebar have magically appeared.