the short answer to your question is, yes. It's a legitimate error. It was struck through grease most likely. unfortunately, they are fairly common and don't command a big $$$ premium.
My biggest problem I'm having is finding a site that lists the errors on all the state quarters. Do you have any recommendations? I have 800+ state quarters.
Although it doesn't carry much of a premium (as previously stated), it's still an interesting find worth keeping.
There is no comprehensive site for SQ errors. The bigger problem is that there are so many errors that are considered minor and not worth more than face value except to some bidiots. Your quarter is one of those. Chris