Why developers should avoid the “just this once” code fixes

I think there is a balance, for example:

We just released a product and there was a huge bug in Safari we somehow managed to miss on a last minute deployment. I found a quick fix and pushed it, allowing people to used the application as expected. Then after I pushed the momentary fix, i went back and ensured it was robust.

Again, if I were writing firmware for a car lets say, the mentality of "quick fix" doesn't really exist. Should be tested heavily.