Small pet peeve: I'm always amazed when I come across different spellings for plurals than I learned when I was in school. I learned the Latin-style pluralization, and those are no longer preferred terms.
personae has become personas
fora has become forums
consortia has become consortiums
You get the idea.
Then I remind myself to repeat the mantra, "English is a dynamic language" and it usually goes away.
[update 14 March]
indices has become indexes