The statute of limitations doesn’t affect an excessive refund claim penalty, according to IRS guidance.

If a taxpayer makes a claim for a refund or income credit in an “excessive amount,” he or she is generally liable for a penalty equal to 20% of that sum. In one case, a taxpayer filed a return more than three years ago that fraudulently claimed a substantial overstatement of income tax withheld and received a refund. The IRS stated in Chief Counsel Advice 202044007 that an excessive refund penalty is warranted. The IRS must generally assess tax within three years after a return is filed but tax can be assessed anytime if a fraudulent return is filed with the intent to evade tax.