Continuity mistake: When the construction worker sees a bird in front of him, he says "Ha!" The worker then sees the city below and falls. At this point, you can see the worker is holding his hat in his left hand, but when he falls through the first plank of wood, the hat is nowhere to be seen.
Continuity mistake: When Bugs arrives on the 103rd floor, he sees the worker on a large plank of wood with some bricks on the other side. The amount of bricks changes between shots.
Continuity mistake: During the seesaw of bricks gag, Bugs takes off one brick, leaving two remaining. After the worker takes off some more clothes, we return to Bugs, and there is now only one brick left.