"Luna and I can stand at either end of the corridor, ... and warn people not to go down there because someone's let off a load of Garrotting Gas. ... Fred and George were planning to do it before they left."
-- Ginny Weasley (OP32)
Invisible gas which apparently knocks people unconscious.
- Fred and George were planning to release Garrotting Gas in a corridor, but left school before they had a chance to carry out the plan. Ginny seized upon the idea as a diversion to keep Umbridge away from her office. NOTE: It seems highly unlikely that Fred and George would have released something which was actually deadly, so we assume that Garrotting Gas is non-lethal (OP32).
garrote = killing by strangulation, or the instrument used for such an act (source: Merriam-Webster)