The IRS has 3 years from the due date of the return (plus extensions) to send you a notice of deficiency. If it misses the deadline, the IRS must prove you filed a fraudulent return. If the statute of limitations is about to run out, make sure they have your current address -- file a change of address form & keep proof.
Also, if you are audited for the same thing 2 years in a row & no adjustments are made, they can't audit you again for the same subject.