First appearance of Liam Noble as Andy-- he will not appear again until Series 8.
The keytar that Jeremy owns is the Yamaha KX5.
A set was used for all scenes of Mark and Jez's apartment beginning with Series 3; the radiator behind the couch looks completely different than the one in Series 1-2.
Mark installs a bracket lock on his door, indicating he has the tools and at least some know-how to be able to fix the bathroom door.
We see Mark and Jez's neighbor standing in front of his door (Apartment 7) in this episode.