It's probably something I'll see all over the place now that I've noticed it once, but I'm pretty sure I had never before seen the last of those singular-via-plural formations until I read a quote by an academic calling some measure of success in Iraq "a positive indice."
"Indices," of course, is an alternate plural of "indexes," and it's not surprising that the kind of people who use the more eggheady-sounding plural for certain kinds of indexes might stop thinking of "indices" as being indexes at all.
"Lense" is pretty common, and it's the most innocuous example, as the pronunciation is the same as that of the correct spelling. "Leave" for "leaf" isn't a mistake you see very often, at least among adults, but it's more analogous to "indice" because of the pronunciation difference.
Tamale, by the way, is another example of this phenomenon (in Spanish, "tamales" is the plural of "tamal"), but I'm less bothered by such transformations when they cross languages. Similarly, I don't see what the big deal is with the supposed redundancy of "Rio Grande River" and "Sahara Desert." (If "ugga-bugga" means restaurant in Uppaduppian and I'm opening an Uppaduppian restaurant, I'm not allowed to call it the Ugga-Bugga Restaurant?)