The speed of rotation of the two hemispherical things on the droid, especially when Rimmer and Kryten first meet it.
Kryten said that his reinstalled file on 'Rimmer' now had pride of place in his database, next to 'Ringworm' and 'Rimshot'. Although if sorted alphabetically, 'Rimmer' would not be between the two words, Kryten doesn't claim it is, contrary to some 'Goofs' claims.
If they took the kidneys from past Lister to give to future Lister, he would not have had any for the insane medi-droid to remove. Though perhaps, the jar of kidneys were from someone else and they only thought they were Lister's when they discovered he had none.