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.

EDIT

I have been asked to clarify the extent of the unreliability. It does not happen with mathematical precision but there has to be some proportionality. For example if I try to light a fire I won't accidentally freeze the entire continent.

Examples of how things might go wrong:

I try to heat up my dinner. I end up with a frozen meal.

I try to repair a broken vase. It breaks into roughly twice as many pieces.

Explanation

The supreme being has tasked sprites with keeping magic balanced. There must be an equal amount of desired results as undesired. The sprites however have limited intelligence and a short attention span. Therefore to make their job easy they apply this 'balance' to each spell as it happens. They do their best either to comply or to do what they perceive to be the opposite. They are not mathematicians, they just do their best to maintain balance.

Magic is fairly small scale. It relates to everyday life. You can't use it to make the Sun disappear or even make a person disappear. The spell has to be 'plausible' and within the capabilities of a sprite..