Nobody has yet mentioned perhaps the biggest cause of level busts: human error. This is when the flight crew set the wrong level in the altitude window, or when ATC want you to climb/descend to level X but actually say level Y and miss the readback.
Phraseology can also play a part. in the Good Ol' USofA "Maintain" also means climd/descend. There have been a few incidents where traffic at (for example) FL390 has been told to maintain FL 350 (ATC slip of the tongue) and rather than querying the difference, down it went, right through the FL370 sitting just behind it!.. But then the Americans have always been a bit different!..
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"Take-off is optional, Landing is mandatory"