From a posting to an online discussion group that I follow:
"…I am defiantly returning…"
And from an online story about insurance companies deploying privatized firefighting crews:
"…(a spokesman) acknowledged that the policies are 'defiantly not cheap'…"
In both cases the writers clearly meant to say "definitely." I seriously doubt they were in a rebellious state of mind (although anything's possible).
In both cases I nearly missed the errors. When a written word has letters missing or is otherwise misspelled, your brain automatically tries to "fix" it. Many times you will end up seeing what the word is supposed to be in defiance of what is actually written.
I hate it when that happens.