On the Custom Map Generation, page 2 of the options menu, the ores menu, hitting the randomization button will not keep in mind the fact that the minimum spawn level must be less than the maximum spawn level and it occasionally makes the minimum larger than the maximum, thus making the world impossible to load.

What I expected to happen was that the randomizing button would automatically make sure the minimum was lower than the maximum.

What actually happened was the minimum was somewhere around 100 and the maximum was around 70 or 50.

I'm not sure how often it happens, but I noticed this because my world wouldn't load after many long periods of multiple attempts.