Well 1st don't feel too bad. I mailed my application off this morning and despite checking and rechecking the form like a paranoid maniac, very nearly mailed it off without signing the check. In fact I had already sealed it in its Express Mail envelope, affixed the mailing label, when I decided to take just one more look to make sure everything was there. Being satisfied I put it into a clear folder, because I was all out of envelopes, and 2 days later (this morning) took one more final look and discovered the unsigned check.
One of 2 things is likely to happen. Either they will accept your application and you will get an RFE or they will reject it and return the form and fees. Refer to page 7 of the N400 guide.
