It is truly amazing the number of people who can't understand that effective communication includes proper punctuation, spelling, grammer and word use. I am a poor speller, but I check my work, and if it is something important, I also have someone else proof read. In a business setting people notice, and make judgements based on writing skills.
Forums like this are more casual, and far more errors can be tolerated. I generally let my spelling errors ride here. I can't figure out how to spell check them, and I don't want to type and use a dictionary.
I've also talked to my son about proper pronunciation. He is an avid reader and is developing a reader's vocabulary. This means that some of the words he knows, he doesn't know how to correctly pronounce. I correct him, and when he says he wants to say it his way, he gets the "people will judge you" comments.
So....pet peeve #367
Penultimate. Used to mean the very best in a sentence like "he was the penultimate player in his league". Unfortunately, penultimate means next to the top, not the top. There is a local movie reviewer who likes this word, but uses it incorrectly every time. I've seen it in his columns repeatedly, and each time it makes me cringe.