What Robin said -- AFAIK, if a someone probed her too deep and perforated her cloacal outpouching, I don't believe it ever grows over and so she will forever probe deep on that side. That's why (again, to reiterate Robin's post) it's always recommended to probe both sides. I seem to recall reading somewhere that that left side of the snake is less reliable and more apt to be perforated, because most people are right handed and may only probe the left side.

I do still probe larger ball pythons. I just don't trust myself to pop reliably. Maybe I just don't squeeze hard enough, or in the right place, or maybe I'm just more comfortable probing than popping since I've been doing it for much longer ... I dunno. I do know that I have one or two known males who are consistently very difficult to evert, so I just don't completely trust my popping abilities to double-check sexes on snakes I'm buying or selling. (And yes, I have been shown how to do it multiple times in person, and I can say I can probably do it reliably about 75% of the time ... But that 25% is the kicker!)

I have heard many say that they believe that popping in ball pythons is more reliable than probing. Animals do have natural variation, and so I think it's entirely conceivable that a female, even a heretofore un-probed female, could probe to a seemingly "male" depth; similarly I would imagine you could have a "short" male. If you see hemipenes, though, it's gotta be a boy. ... And in theory, if you pop properly and DON'T see them, it's gotta be a girl ... I just don't trust myself that it isn't just a really strong boy who doesn't feel like showing 'em

I do think that there's some truth to the idea that it isn't "proven" until it's proven, though. I had a "male" blood python who flashed "his" hemipenes at me constantly lay eggs one day ... Granted, balls aren't bloods, and I think in this species if you see what you think are penes it's probably safe to call "male." (If anyone has experience to the contrary, though, please let me know!) However, even with all the probing and popping in the world, I still don't know if you can definitively call a ball a female until she's produced eggs ...

Which could bring up another topic of debate (maybe better suited for the business section?): can you truly "guarantee" sex of a female ball python?