Insanity

Insanity was a condition of mental instability in sentient beings. It was colloquially referred to as madness or going crazy.

The supercomputer Xoanon was inadvertently driven insane when the Fourth Doctor helped repair it. (TV: The Face of Evil)

After falling into an abyss on Kastria, the version of Eldrad that was regenerated from the hand was driven mad in his solitude. He talked to the dust that surrounded him - the remains of long-dead Kastrians. (AUDIO: Eldrad Must Die!)

A psychic locket showed Alex Hopkins the future, and also drove him insane. (TV: Fragments)

Dalek Caan became insane after he saw time itself. (TV: The Stolen Earth/Journey's End)

Vincent van Gogh suffered from insanity near the end of his life. (TV: The Pandorica Opens)

Giuseppe di Cattivo was driven insane after painting The Abomination. (TV: Mona Lisa's Revenge)

The Doctor, on numerous occasions, has implied that he himself is slightly mad no matter what regeneration his own; others have called him crazy. (TV: The Eleventh Hour, Flesh and Stone, The God Complex, The Day of the Doctor, etc)