You can't rely on your spell checker because it isn't designed to distinguish between the word usage and its intended definition. For example, words such as "their", "they're" and "there", regardless of the intended definition, will always be accepted by your spell checker.
Other common words that are misspelled and misused are:
which and witch where, we're, were of and off your and you're its and it’s
There are many others but that's just to give you an idea of words that can make or break your writing.
