Thi's user know's that not every word that end's with s need's an apostrophe and will remove misused apostrophe's from Wikipedia with extreme prejudice.
their there they're
This user thinks that there are too many people who don’t know that they're worse than their own children at spelling!
your you're
This user thinks that if your grammar is incorrect, then you're in need of help.
its & it's
This user understands the difference between its and it's. So should you.
to too two
This user thinks that too many people have no idea how to use words that they should have learned in grade two.
whom
This user insists upon using whom wherever it is called for, and fixes the errors of whomever they see.
if & whether
This user knows how to use "if" and "whether" correctly.