I guess the worst offence is the one that most ppl get wrong. Even BBC reporters, presenters, etc regularly get this one wrong. Ignorance of the I/me/myself/ rule is now almost universal, to the extent that:
1. When I once corrected an Aussie colleague, she insisted the correct form didn't sound right, so I must be wrong. She would therefore continue to use the form she heard everyone else use, and since they all agree, they must be right. I wasn't looking for an argument, so I simply let it go. This battle simply isn't worth the candle.
2. When my daughter (who works in publishing) pointed out numerous infractions of the rule in one of her company's publications, her much older bosses dismissed her 'corrections' as baseless. When the department head noticed the same errors some weeks/months later, those same bosses were forced to take note this time, ate some humble pie, and apologized to my daughter for being so dismissive of her proofreading prowess. 'You were right after all Stacy......well done'. Yeah right, well done, after you let several issues go out that proved you were a bunch of incompetent (choose your own expletive here) who were promoted well above your ability.
It's now reached the stage that on the rare occasion I hear someone on the radio or tv get it right, I shout out an audible cheer, along the lines of 'well done mate'. So what exactly am I referring to? (No scolding me please for ending a sentence with a preposition - I know the rule, and I know when to break it.) Here is a link that explains all: