Android In-App Billing failure, saying "You already have a pending order for this item."
While there are issues with in-app billing like the links presented here (i was active commenter on issue 39), there are some worthwhile things to check on as well that can cause a 500 response from google.
Namely, "restore transactions" and abusing that call. Abuse of that call is done on a per user basis and i've seen a google account get blocked with a 500 for a few days, which can certainly happen during development to you depending on how you implement.
Best advice here is to expand your testers to more users (or devices with different primary google accounts), running different versions of the android market. If you can get one positive response from someone, then more than likely you're good to go and the rest is at google's door.