Episode #4.4 - S4-E4
Revealing mistake: When Mr. Baxter punches Mr. Hicks you can see that Baxter's fist passes in front of Mr. Hicks face and dosen't actually hit him. Also Hicks' reaction is too late, and his head moves after Baxter's fist has passed him.
Revealing mistake: When the drunk Andrew is asleep in the boys' toilet cubicle, the wall shakes when Tommy shuts the outside door when Alan and Justin leave.