It drives me crazy...

...when a TV show or movie sequel replaces an actor but gives no explanation as to why the character suddenly looks completely different! 
Example: Evie in the third Mummy movie, Daisy in the Dead Like Me movie. 
I'm not a complete idiot! I can tell when I'm looking at a different person!
At least in The Matrix, when they replaced the Oracle's actress, there was an explanation! (Also, I make exceptions for actors dying).