The Tenth Doctor meets one of his previous incarnations.
Did You Know?
In interview with Doctor Who Magazine that was released prior to the broadcast of Time Crash, writer Steven Moffat
confirmed that "Time Crash" is considered part of the official Doctor Who canon. (There is a cutaway at the end of the preceding regular episode, "Last of the Time Lords" which allows for "Time Crash" to occur between that episode and "Voyage of the Damned".) See more
[observing the Fifth Doctor
Oh! The back of our head! Sorry... sorry... it's not something you see every day, is it? The back of your own head? Mind you I can see why you wear a hat. I don't want to seem vain, but could you put that back on?
Followed by Doctor Who: Devious