A common #mistake is to write the word of instead of have or the contraction 've:

I could of told you that.

instead of:

I could have told you that.

or alternatively:

I could’ve told you that.

The reason for the mistake is that the pronunciation of have in unstressed contexts is the same as that of of, and the two words are confused when it comes to writing them down. The error was recorded as early as 1837 and, though common, is usually considered unacceptable in Standard English.