Are you guys sure it's not just the frog shedding. I've had them peel up tip-first and then I cut off the flap (because I don't want to peel off the whole frog) but they do shed maybe once a year and that would explain why it's happening on multiple feet.
The only thing that doesn't jive with that theory is that he is lame. He shouldn't be lame if it's shedding naturally. But I could totally see that if the frog is shedding, someone (like myself) cutting off the "tab."
There's a lot of stupid out there!