My favourite is the 1986 European Cup Final. With the game drawn and going into ET, Venables takes him off. Best player Barca had, the only one likely to break the deadlock, but no Venables takes him off. He was probably trying to make it clear who was boss, which was the ongoing problem.
Problem is, as well as the most likely game-changer, Schuster was their designated penalty taker, the one you would give the all important first peno to... He was German for Christ's sake!... and he takes him off the pitch. Nutter. Schuster leaves the pitch in a strop, then the stadium and gets on a cab. The Sevillan cabbie looks through the rearview mirror and turns around, completely startled: "aren't you supposed to be in there playing?". Schuster's face must have been a picture.
Cabbie drives off and with the radio on Schuster listens on, probably with a cnutish knowing smirk on his face, as Barca miss all four of their penos (the worst part being Steaua also missed their first two, so getting the first one in would have made a massive difference). Of course, stupid as Venables was, Schuster made him the winner on the day with his actions. Venables' and the Board's problem was the fans loved their temperamental genius, but walking out on the team like that broke the camel's back, which led to Venables being able to have him spending 1986-87 playing reserve team football.