In my world, magic is unavoidably unreliable, for example:

You have a puncture. You cast a spell to mend it and with equal probability either it is mended or another tire is punctured.

You are trying to save an injured person. You cast a spell. Either they are saved or you kill them.

Question

It seems to me that such a magic is completely worthless. Can anyone prove me wrong by suggesting a case where both the intended result of a spell and its opposite would both be advantageous?

Note

There is no way to make magic more reliable. It is just a fact.