That poem always amuses me... I want it to be:
"i before e, except after c ... apart from words like science .. and foreign, and reign, and neighbour".
Just found a link to this, which is catchier than mine

:
The most irritatingly common grammatical screw-up is the confusion between "its" and "it's" (like in Paul's post about the misspelling, for example

).
"Its" - possessive adjective.
"It's"- contraction for "it is" or "it has".
Cheers,
Refractor.