Time ram

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, describing it, said it would cause a black hole strong enough to destroy an entire universe. (DW: Time Crash)