Continuity mistake: After Gareth hits Tim on the head with his newspaper the following shot shows that the briefcase and newspaper have swapped hands. (00:04:25)
Continuity mistake: In the scene with David and Gareth in David's office, David gives a hole-punch to Gareth as a souvenir. The hole-punch has a guide accessory sticking out of it (to punch holes in the middle of different sizes of paper), which is adjustable. At the beginning of the scene it only sticks out by about 2 cm, but by the time David hands it to Gareth it is about 5 cm - even though no-one has touched it. (00:15:35)
Continuity mistake: When Donna comes in late and David is asking her where she has been, as it changes shots between Donna and David, in the background Dawn's right sleeve changes from being down to up at wrist height several times but she is in the same posture all the time.
Answer: It's because he keeps doing it, like clockwork, on exactly the same line and he has such a blank, serious expression on his face that Tim (Martin Freeman) can't help but laugh.