In my original army list, the HQ choice was a Daemon Prince called Felltoade who bore the Mark of Nurgle. Upon reading the special rules for Plague Marines, I fell under the impression that, should the army be led by commander bearing the Mark of Nurgle, Plague Marines could be taken as either an Elites choice or a Troops choice. I promptly added two squads of Plague Marines as Troop choices and didn't give it a second thought.
Then I reread the rules: nowhere does it say "commander." It says "Chaos Lord." I made the mistake of interpreting "Chaos Lord" to mean commander in the Plague Marine rules, and, in so doing, saw the rule as stating, for the most part, that "if your army is led by a commander bearing the the Mark of Nurgle, Plague Marines could be chosen as either Elites or Troop choices."
Oops. Crap. Dammit.
So, now I got a Daemon Prince built. A big one, too, and heavy... probably well over a pound in weight and based on a Tyranid Carnifex. Felltoade may be one of the best daemons I ever built. Oh, well. He'll be popping up, eventually. My plan is to take a Chaos Lord or Daemon Prince as an HQ choice if/when I reach the 3000 points, 5000 points, 7000 points, etc., size, so I'll probably have him leading the 3,000 point increment. (If the army ever gets to be 7000 points, I'd probably sell it on eBay.)
Replacing Felltoade as army commander is a conversion based on the Chaos Lord standing on a Space Marine's helmet, bearing Nurgle's Mark, and his retinue of Plague Marines. Modelling-wise, I did a lot more work on the Daemon Prince than on the Chaos Lord, but I'm pretty pleased with the results, in both cases.
(And, sure, I could have kept Felltoade in the line up, but then I'd have to figure out points for two new Troops choices, and I'd rather have Plague Marines as part of the "core" army rather than plain old Chaos Marines, anyway.)