Board Thread:The Reference Desk/@comment-5545417-20131116122722/@comment-2378-20131130155750

Similar to The Empire of Death, it is mentioned in the short story Briefly Noted that the Second Doctor forgot most of the events of The Three Doctors after being returned to his proper time, though he did remember that he lost his recorder.

In The Five Companions, the Fifth Doctor told Ian, Steven, Sara, Polly and Nyssa that their memories of visiting the Death Zone will probably fade with time so the same is presumably true of the earlier Doctors and the companions in The Five Doctors. It would also explain why Sarah Jane doesn't remember the events of that story in School Reunion.