The Eleventh Doctor once got bored and shaved his head. This allowed him to wear a wig and hide a TARDIS key in it that Tasha Lem could not discover when he set foot on Trenzalore, which had a technology ban. This wig was presumably destroyed by his regeneration into the Twelfth Doctor. (TV: The Time of the Doctor)
Behind the scenes Edit
When the wigs constructed for the guest characters in The Two Doctors were lost in transit, the actors had to be fitted for new wigs in Seville. These Spanish wigs were subsequently used for the studio shoot in England in order to keep continuity.
Sylvester McCoy has the distinction of having played two incarnations of the Doctor; when Colin Baker declined an invitation to record the regeneration scene in Time and the Rani, McCoy put on a wig and Baker's costume and, with his face obscured by special effects, filled in for the role.