Continuity mistake: The top of the cabin is blown off when Agent Al gets "taken up". However, in the overhead shot that immediately follows, we can distinctively see all the figures on the floor of the cabin, including the agent (who is still tied to the chair).
Continuity mistake: When the alien screams in the warehouse and the glass breaks, immediately after it shows a closeup of his face and the glass is still intact.
Continuity mistake: When Telly runs out of the house after being told by her doctor and husband that she never had a son, she is wearing a long black cardigan. This is shown in scenes of her running down an alley, as well. Shots after that, for the balance of the movie, show her in a short dark jacket.
Continuity mistake: When Telly is walking on the bridge and stops to look at the rooftop playground there are two different bridges. The aerial shots show a much wider, more open walkway than the shots from the bridge. Note how wide the concrete squares are in relation to her body. In the aerial shot the middle seam between the squares is just a crack, but from the bridge you see that the seam behind her is smoothed over by cement or similar. (00:34:40 - 00:34:50)