This event occured during Episode 5: All Bets Are Off

During their adventure in Las Vegas 1976, the Doctor came to realise that there was something strange about Terence, but the precise nature of it is unclear.

Terence is either a fixed point in time and shouldn’t be, OR… isn’t a fixed point in time and should be.

This is obviously wrong because fixed points in time are usually events that the time stream declare MUST happen or exist, no matter what. This means that if a fixed point in time were altered, it could be very, very, very bad!