The way the keys are moved around will probably mess me up more than anything.
For what it's worth, when I switched to the HHKB about two months ago, the placement of backspace was trivial to adapt to; took less than ten minutes for it to feel totally natural (for me, it actually triggers a reaction of "Why don't all keyboards place the backspace key there? It's so much better!", the same as putting Control where the Caps Lock key usually is.
The really problematic keys for me were (and still are) '-' and '+'. I still often have trouble finding them today. Which is weird, because physically they're in the same spot on the keyboard as on more standard layouts. But for some reason, I have trouble hitting the correct one when I'm touch-typing quickly on the HHKB, maybe due to having two full-sized keys to their right, rather than a single 1.5x key.
After adapting to the HHKB, when I need to type on a keyboard with a more conventional layout (say, on a co-worker's computer) the '\' and 'Backspace' keys are the keys I have trouble with. Not at all unusual for me to discover that I've accidentally inserted a string of \ characters where I meant to be backspacing. I find I just have to be extra-mindful, since those characters are now in my muscle memory from the HHKB layout.
The HHKB also kind of reset my expectations for keyboards; my trusty old Filco tenkeyless (which I've used for years) looks huge, inelegant and, frankly, a little absurd to me now. It now spends most of its time hiding behind my monitor, and only gets pulled out and dusted off if I don't have the HHKB with me, which is becoming increasingly rare. Might be worth sticking with full-sized or tenkeyless keyboards if you don't want your brain to rebel against them.