I remembered another one after writing a reply on a different thread... "Can" and "May". "Can" means to have the ability to do something whereas "may" means to have the authority or permission to do something. My friend's father used to bug us all of the time when we'd call. If we asked "Can I speak with Jason?" he'd always reply with "I certainly hope you can," and would just stay on the line until you actually asked the question with "May" instead.
And as I was writing this reply, I noticed another one that many misuse / misspell... "used to" and "supposed to". Often times, people will say (or write) the verb without the "ed" at the end even though they usually are using the phrase in the past tense (or at the very least, present perfect tense which would also require an "ed" ending as in "I have finished,"). The problem is that the "ed" sound from the verb gets mixed in with the "t" sound of "to" and so the person won't use the "ed" at the end of their verb.
One last confusion that I see often (esp. when I was interviewing for jobs) is when they'd say "We're happy about you joining our company." Here there isn't necessarily a grammatical error; the problem is really about what semantic they are trying to convey. It is like the previous example "It's time to eat Grandma." Technically it is grammatically correct whether you have a comma in the sentence, but the semantics of the sentence change depending upon which version of the sentence you use. Similarly, by writing "We are happy about you joining our company" means that they are happy about you personally. If you were a top-notch person in your field, then this is probably what they mean; were Bill Gates to join Apple, then this sentence is probably what they want to convey. However if the applicants for a position are very similar, then it is not necessary you they are happy about, but rather the fact of you joining the company. In that sense, the correct sentence should be "We're happy about your joining our company," where "your" is the possessive of the noun "joining" that is really what they are happy about. Similarly, an interviewer usually means "I am excited about your inquiry into this position" and not "I am excited you inquired into this position." They are more excited about the inquiry rather than that the inquiry was made by you personally.