a phrase a day — double edged sword

The phrase “double edged sword” is a metaphor used to refer to something that can have both favorable and unfavorable consequences. Technically, a double edged sword can hurt you no matter how you wave the sword. It’s a deadly weapon that can cause death or injury. I don’t understand how a sword can bring favorable consequences. Nonetheless, the phrase is now used to refer to something or a situation with both positive and negative effects.

Can you think of an imperfect metaphor like this one that somehow sounds illogical?