You answered your own question. If by it can happen to anyone, you mean your knee can suddenly violently spasm just as you're stood behind someone, you would be correct. However, I somehow doubt that is what happened to Terry. Most people have at least a double digit IQ that can decide that kneeing someone in the back in a CL semi-final is a stupid thing to do and puts your team in an awful lot of trouble for no reason, alas ol' John decided against this.
And I don't really care for the argument you're having with Frankie or whatever, I just don't see how you can justify what Terry did as can happen to anyone. No, you have to be a special kind of moron for it to happen to you.