Here are three common mistakes: 1) Misspelling one of the most frequently misspelled words (because, a lot, separate, definite, etc.). 2) Using the wrong homophone. Examples include using to instead of too, or there instead of their, and so on. 3) Incorrectly using apostrophes. Often you'll see signs for Sub's on Sale, or Buy Car's Here. These words should be plurals, not possessives. No apostrophes are needed.