"Should of"
For the love of God, it's SHOULD HAVE. Yes it "sounds the same", but just THINK for a second and realize that "should of" DOESN'T MAKE ANY SENSE. After "should" there must be a VERB: should have... should write... should eat... should wash... should die... should drink.. etc etc etc. "Should of" makes ZERO SENSE. If your brain is incapable of realizing that "should of" makes NO SENSE, I don't think you should be writing ANYTHING AT ALL.
I know, I know, I'm bracing for all the "should of" jokes in the comments. I "should of" seen that coming, right?
EDIT: Incapable OF!