Factual error: The polymorph goes through a metal pipe and a tap which bulges and becomes mis-shapen. The tap then changes back to its original shape, which is impossible. (00:18:00)
Timeslides - S3-E5
Continuity mistake: Kryten needs to ask what a pub is, yet in the episode "Backwards" he and Rimmer worked in one for three weeks as the comedy act "The Sensational Reverse Brothers", and, later, were involved in a brawl in the same pub. Originally, the episodes were to be shown in a different order, which explains why the mistake occurs, but doesn't excuse it. (00:12:55)
The Last Day - S3-E6
Plot hole: In this episode Kryten has no idea how to operate a bazookoid, yet earlier on in 'Polymorph' he was able to change the settings of one from standard to heat-seek and fire it without a problem. (00:24:45)
Visible crew/equipment: When the bazookoid blasts hit the Polymorph near The End, you can see the stand that's holding the Polymorph at the bottom of the screen. (00:24:45)
Timeslides - S3-E5
Continuity mistake: When Rimmer visits rich Lister, he apears from a point right next to him. But when he leaves, the point at which he disappears is much furthur back.
Continuity mistake: One of Starbug's legs breaks off when it hits rocks on the moon but in the next shot, the leg has returned. (00:23:20)
Timeslides - S3-E5
Continuity mistake: When we see the photo of the skiers, there are no boxes in front of the screen. When the shot changes, two boxes have appeared which Rimmer and Lister use as steps. (00:07:20)
Other mistake: The milkshake and toffee crisp come out of a different vending machine to where they were ordered. (00:01:50 - 00:06:55)
The Last Day - S3-E6
Continuity mistake: When Kryten takes the invitation from the skutter, he picks it up with his right hand. In the next shot it has jumped to his left hand. (00:10:20)
Timeslides - S3-E5
Revealing mistake: When Rimmer dies at The End of the episode, a dummy is visible just before the explosion. (00:26:40)
Timeslides - S3-E5
Continuity mistake: During the golf game, Lister's locks move between shots throughout the scene. (00:01:20 - 00:02:20)
Timeslides - S3-E5
Audio problem: In the photo of 'Smeg and the Heads' you can see that the drummer frantically plays the cymbals, but no sound comes from them. (00:11:40)
Timeslides - S3-E5
Factual error: The newspaper detailing the bomb attack on Hitler lists the date as "Tuesday May 4 1938." May 4th, 1938 was a Wednesday. (00:09:15)
Revealing mistake: When the Polymorph, posing at the snake, jumps on Lister, it is apparent that the snake on Lister has been replaced by a plastic inflatable replica that has no features and does not move. (00:10:05)
Timeslides - S3-E5
Plot hole: In S2 E2 Better Than Life, Rimmer mentions that his brother Frank was 6' 5" by the time he was 11 due to their father pulling him on a tractor, however in S3 E5 Timeslides, the picture of Frank Rimmer's wedding shows Frank to be of fairly average height, and not 6' 5".
Answer: The cello is a large four-stringed instrument, which, when it is played, stands vertically on the floor between the player's legs (assuming they are seated). If it is to grow as large as the other instuments mentioned will, it would require a rather unseemly lack of femininity to be able to encompass it with the legs.
Rooster of Doom