Character Sacrilege = Something that would cause character exit, or make them no longer fun to play.
Example: Let's say your character was out to make it big, be rich, famous, and living the high-life. Character Sacrilege would be... succeeding at his dreams. Once he had those things, he wouldn't go out and risk his neck any longer, you could theoretically change the motivation to something else, but that would also change the character. In stead of being the money-hungry morally ambiguous individual, it suddenly changes him to something else and therefore would ruin the character entirely.
Another example would be like playing a character with a special personality... and then having them face something in which they MUST compromise (often times through some sort of genius paradox) or have some sort of other role thrust upon them. (Self defense, for some reason, rarely makes this happen) so, it could be some sort of skill, obligation, trauma, or other aspect that sours the fun of the character to the player.
A really good example are these badass warriors that a GM then introduces a love interest into the plot. The players rarely want to see their spiky-armored bundle of muscles doing nice things for his snuggle bug.
Here is one that has caused many a character suicide... You make a bikini model and then have her suffer some sort of scarring injury. Players will hurl these sex symbols into swarms of orcs and undead (without any regular defenses) like no berserker ever would.
All of these are examples of Character Sacrilege.