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. (DW: The Time Monster, Logopolis) This happened to the Tenth Doctor and the Fifth Doctor. The Fifth Doctor, decribing, it said it would cause a black hole strong enough to destroy an entire universe. (DW: Time Crash)
