I am having the same problem. After much searching, I'm relatively certain that gridlines 1-4 and A-G don't actually exist in my project. However, I cannot use these numbers or letters in my grid - I get the same error message as noted by Todd V. above, "You have entered a label value that is already in use." The only solution I can find is to convert the smart grid tool to objects - give it a lobotomy, as my coworker says - which removes the value of having the tool in the first place. Any help would be greatly appreciated!