One of my major pet peeves is the misuse of should have. I frequently hear people say and see people write should of instead. I think this may stem from the contraction should've, which sounds like should of when said aloud. Go ahead, try it. Okay, that is all about that one. It's just wrong. Don't use it.
Another one that drives me up a wall is using the word ran when it should be run. This seems to happen most often when the word is paired with have. Many people say/write: "I should have ran," rather than "I should have run." Incidentally, that is the correct usage of should have. Ran is the plain past tense of the word run. Run is used in instances of had run, have run, and will have run.
It amuses me when people try to use a common phrase, but actually say the exact opposite of what they mean. For example: "I could care less about that movie," when they actually mean "I couldn't care less." Could care less isn't the phrase anyway, but it also does not mean that the person doesn't care at all. That's what the phrase is supposed to mean. Another example of this is someone saying: "I don't care to go to the movies," rather than "I don't care if we go to the movies." Don't care to means you don't want to do something; whereas don't care if means you don't mind doing something.
Please, think before you speak! And look twice when you write!