There are many common mistakes.
Many native English speakers don't know the difference between;
lay and lie (While lay is the past tense of lie, lay is also the form used when the person performing the act isn't the subject of the act. You should lie down, but yesterday, you lay down for a nap. Lay the letter on the table.)
then and than (Then refers to time; than is used to compare. He has more marbles than I do. First, you turn right, then you turn left.)
affect and effect (Affect is a verb unless you're referring to someone's demeanor. Effect is used as a noun only in cause-and-effect situations. How will this affect our budget? I can't see that it will have much of an effect.)
a lot and allot (to allot is to assign an amount. A lot means many.)
'til and till (I've found this mistake in published books, too. Till is an agricultural term. When you're shortening the word UNTIL, you use 'til)