Example of a critical glitch that still succeeds:
You are trying to save your mother from the razorboy sent to assassinate her. You attempt to shoot them in the head, but roll a critical glitch. Upon spending edge, you succeed in the roll.
Because you succeeded, you shoot the razorboy in the head.
Because it was a critical glitch, the bullet went through your mother's head first.
A drastic example, and your GM is a dick, but it can happen.
~R~