In the Empire Diner, Igby is in the same position in his seat and plays with his straw the same way throughout the entire scene except in the one far shot where his glass is pushed aside and out of reach (from his changed position). The salt and pepper shakers also move around (in some shots they are closer to Sookie, in others, to Igby) and seem to be absent in some shots.
When Igby is outside Sookie's apartment and they are talking through the door, we see Sookie sitting up against the door as Ollie walks towards her to open it, and milliseconds later he opens the door with ease as if she weren't blocking it at all.
When Oliver comes into the loft and finds Igby and Sookie sleeping side by side, Sookie sits up slightly and the camera makes a quick cut to Oliver. The camera quickly cuts back to Sookie and Igby, and Sookie's head is back on the pillow again (as in the first shot) and then she sits up again.
When Sookie meets Igby in the street and invites him to the park, it's very overcast and looks like it's starting to snow. When they're in the park in the next scene, it's much brighter, almost sunny, and there's no snow.
In the opening scene, Ollie says at one point, "It's almost six." The scene takes place in the fall, so by then it would be dark outside, but it's as light as earlier in the afternoon in the same scene.
Despite talk throughout the film of a passage of time, the coming of summer, the advent of fall, and the approach of Christmas, the entire film was clearly shot over a very short time in the fall. In the scene outside the campus where DH tells Igby his mother is having a mastectomy, for example, DH talks about staying with him "this summer," yet there are dead leaves on the ground and bare trees.
The goof items below may give away important plot points.
At the start of the film, the spoon with the strawberry poison concoction falls to the floor of Mimi's room bowl-side down. At the end of the film, the spoon is seen on the floor of the room bowl-side up.