Time ram
From Tardis Wiki, the free Doctor Who reference
Time Ram is the name given to what happens if two TARDISes attempt to occupy exactly the same time and space. (DW: The Time Monster)
It can be narrowly avoided by the two TARDISes materialising within each other. : (DWThe Time Monster, Logopolis) This happened to the Tenth Doctor and the Fifth doctor (DW: Time Crash