I am definitely irritated by the frequent replacement of definitely with defiantly. As far as I can tell, this is simply a matter of people not paying attention when they're typing. I would hope that people know the difference, though that may be asking too much. It's one of those errors that is easy to make if you don't pay attention to what you're writing or typing.
Another common error in both speaking and writing is the use of the incorrect past participle form of certain verbs, those used with have. There are many verbs that have unusual forms for the past participle. For example: have sung, have swung, have come, have begun, have done, have drunk, have eaten, have gone, have swum, and many more.*
These are verbs that just have to be memorized because there is not necessarily a logical or consistent means of figuring out what the past and past participles should be. Many people, however, just use the regular past form with have, rather than the correct past participle. They say: I have sang a song; I have swang a bat; I have came to school; I have began a book; I have did this; I have drank pop; I have ate pizza; I have went to the store; I have swam in the pool.
Actually with swang and swung, more commonly it's actually the past pariciple used in place of the past tense: I swung the bat.
These are further confused by those that don't follow the pattern. It is drink, drank, drunk; but not think, thank, thunk. Sing, sang, sung; but not bring, brang, brung. That one IS frequently used by people trying to genralize the rule, but failing miserably.
English is a difficult language and it is distressing that more and more Americans don't even know how to speak or write it correctly. Or they don't care, which is almost as bad.