There are bugs and there are bugs, some of them more serious than others. This one is an effect of the ambigous mapping between cancelling a task, default choices and the escape button:
In Font Book, choose a list of fonts containing at least one system font and choose disable. A dialog will appear to inform the user that there are system fonts among the chosen, and disabling them may not be a good idea. So the user's left with the following two verbs: cancel and disable - cancel being the default choice of throbbing blueness.
As for me, I thought "I don't wanna do that!", instinctively hitting the keyboard's escape key. The effect: disable...