The only thing keeping this industry from raining down a torrent of borderline fraudulent cases (or at least procedurally-suspect cases) is probably the fact that the public would close these loopholes if they knew they existed. They rely on the sentiment that they're just going after deadbeats with legitimate grievances, instead of sticking their hands into the pockets of two-and-a-half generations of underpaid workers who are perennially hit with economic crises.
Ironically, the statute of limitations (on debts, not FDCPA claims) is part of what encourages this bad behavior; debt buyers would rather file on time and ask for forgiveness of their procedural oversight, than file past the statute of limitations and hand the alleged debtor a prima facie meritorious defense. The problem is that judges let them get away with it. And so goes the legitimacy of the rule of law for a vast swathe of the nation, for the sake of a few Benjamins paid out to people who didn't even own the debt in the first place. But then, they did get away with it with houses 20 years ago.