I always thought that "sic semper tyrannis" meant "death to tyrants." I learned this, as I imagine pretty much everyone else did, in reference to the assassination of Abraham Lincoln.
You too? Yea, it was probably because that is what you were told repeatedly in school by a history or "social studies" teacher who didn't really know much about history and didn't care to learn anything outside the textbook. It really means "thus always to tyrants" which I learned from the very enjoyable film, The Conspirator. It is also the motto of the state of Virginia, an interesting fact that I think could have been thrown in to the lesson plans. Before you get ahead of yourself, I don't think there has been a conspiracy to modify the meaning behind Booth's proclamation, but we do ourselves a disservice if we perpetuate this myth for another generation.