Don’t cry over spilled milk, it is just a minor mishap — a misfortune. If the surgeon operates on the wrong leg when you go in for knee surgery? Now, that’s a mishap to get upset about.
You might assume that hap is short for happening, but it is actually an Old Norse word for "luck." So with mis ("bad") in front of it, it literally means “bad luck.” Mishaps can be accidents of great consequence or minimal consequence. People often say that an event went off “without a mishap,” meaning that it went smoothly. Hopefully this will be the case if you ever get knee surgery.