Homeless for the Holidays - S3-E10
Continuity mistake: Nick the dog is standing in the sink and Mark and Doug have got him just about completely covered in soap. The dog jumps out and runs out of the room - by the time he reaches the hall, only a couple of seconds later, the soap has almost completely disappeared.
Answer: It was in "Chaos Theory", the first episode of season 9 (time code 00:13:55).
Super Grover ★