Goodbye, Sarah Jane Smith: Part 2 - S4-E12
Revealing mistake: In the shot in Sarah Jane's attic right after Ruby is thrown into the sun, the group are talking in the attic and to the left of the screen, you can see that the blue screen used to create Mr Smith is visible.
Mona Lisa's Revenge (2) - S3-E11
Plot hole: At the end, all the things and beings that manifested from paintings or drawings are pulled back into their paintings or drawings, including K-9 from Clyde's book. But in the last scene in the Vaults, the puzzle lock can be seen still attached to the Abomination's case, even though it was manifested from a drawing in Clyde's book. The crew probably forgot to remove it after the scene where the paintings went back.