Under the Employee Retirement Income Security Act, Form 5500s are due to the IRS the last day of the seventh month after the plan year ends, according to the agency's website.

So for the plan year ending in 2011, the due date would by July 31 of the next year for a calendar-year plan.

Read: 5 year-end items for all plan sponsors to check

The DOL and IRS try to discourage late filers by imposing steep fines. The IRS penalty is $25 per day, up to a maximum of $15,000. That would mean 600 days worth of fines.

For the DOL, the penalty is potentially far more severe. It can run up to $1,100 per day, with no cap on a maximum penalty.

Those penalties present the potential for some massive punishment. More than 1,100 days have passed since July 31, 2012—the due date for plan year 2011 filings.

Conceivably, plans delinquent for that entire period could face fines that surpass $1.2 million.

But the IRS and DOL are not without a more merciful side.

The DOL's Delinquent Filer Voluntary Compliance Program significantly reduces delinquency penalties if late filers submit the Form, along with those reduced penalties, before the DOL notifies the plan administrator about the delinquency.

Which is to say, if a plan turns itself in before the DOL catches the delinquency, a lot of money can be saved.

Just how much? For small plans with fewer than 100 participants, the fine is $10 per late day, with a $750 cap.

Large plan filers also pay a fine of $10 per day, with a cap at $2,000 per benefit plan.

There is one important "wild card" in the question of whether a sponsor, who might have been under an honest but mistaken perception that it was not required to file a Form 5500, would be eligible for relief under the Delinquent Filer Voluntary Compliance Program.

Plans that receive a late-filer letter from the IRS are still eligible for the relief under the program.

However, if a plan administrator fails to self-correct the issue before receiving DOL notice, they are not.

If a sponsor were to discover that it should have filed when it had not, why wait and take the chance of hearing from the DOL first, potentially eliminating eligibility for the fine relief under the delinquency program, asks Christopher Allesee, an attorney with Graydon Head and Ritchey.

In a blog post, Allesee says, "if you do not file a Form 5500, take a second look at your health or welfare plans to make sure they are not subject to ERISA, or that your plan meets an exception to the filing requirements."

He adds: "For any filings you have missed, consider submitting under the delinquent filer program as a proactive measure, or you may wind up explaining your actions to the IRS and might even lose out on the chance for a far cheaper resolution through the DOL's delinquent filer program."