Mondegreens and malapropisms can be fun but i do get irked by some of the common grammar mistakes that people make. It's already been mentioned but saying 'should of' instead of 'should have' or 'should've' really annoys me. For some reason writing 'wandering' instead of 'wondering' also annoys me, as in 'i've just been wandering if aliens exist'. Often confused are the words 'exasperate' and 'exacerbate' but perhaps that's a little more understandable.
When i was a kid i thought the lyrics to the wizard of oz song were 'somewhere over the rainbow weigh a pie'