I don't make it a practice to seek out writing errors, they naturally jump off the page screaming, "Pssst, hey, girlie! Lookie here. I'm a gross error, don't you see me?"
The most common error is confusing your for you're, were or we're, or there, their, and they're. There are many more but, these are blatantly obvious and, today's technology makes it almost impossible to realize these words are being used incorrectly.
Confusing Words.com is a great website where you can quickly determine the correct word to use. The words are grouped according to the way they are most often confused or misused. You type in the word you are unsure about and click find to get your results.
If you prefer the direct link (to copy & paste), it is - http://www.confusingwords.com/index.php