Is it worth pursuing a $40K error if it is 5 years old?

Found a significant rate error on an Ameren Illinois account — about $8,000/year in overcharges. The error goes back at least 5 years based on available data. Illinois backbilling limit is 2 years, so the utility will probably only refund $16,000. But the total overcharge is closer to $40,000. Is it worth fighting for the full amount or should I take the 2-year refund and move on?

Take the $16,000 first — get that locked in so the client sees immediate results. Then decide if the additional $24,000 is worth the fight. Filing a complaint with the ICC takes time and there's no guarantee they'll extend beyond 2 years. If this is a larger client with ongoing audit potential, the $16,000 win builds trust and opens the door for future work. If it's a one-time engagement, maybe the complaint is worth pursuing. I'd say the break-even on filing a complaint is around $15,000-$20,000 in disputed amount — below that the time investment doesn't justify the uncertain outcome.

Phil gives sound practical advice. I'd add one consideration — check the Illinois contract statute of limitations which is 5 years for written contracts and 10 years for some claims. The 2-year ICC backbilling limit is a regulatory policy, not a hard legal bar. If the overcharge total exceeds $25,000 in disputed amount, it may be worth consulting with a utility attorney about whether a civil claim has merit in addition to the regulatory complaint. I've seen attorneys take these cases on contingency when the amounts are large enough. But for most auditors, securing the 2-year refund plus correcting the rate going forward is the practical win.

Going to take the 2-year refund and correct the rate first. That's $16,000 recovered plus $8,000/year ongoing savings. If the client wants to pursue the remaining $24,000 through the ICC or civil claim, I'll support them but I'm not going to delay the guaranteed recovery for the uncertain one.