WP7 app claims a request to my GAE service produces a 500, GAE claims a 200, where to start?

I am writing a Windows Phone 7 app that interfaces with my GAE backend. For a certain request, the WP7 client claims a 500 error is returned. I see that request in my GAE logs plain as day, correct contents, time, user agent, everything m开发者_JS百科atches and no errors of any kind, the resulting http code is 200, according to the logs. But the wp7 app sees a 500.

I don't know where to start - I don't see how the GAE logs could be wrong, but I don't get how the WP7 app could be wrong either. I'm not running fiddler or anything of the sort.

I don't know where to start with this one...

---

Problem solved, apologies to WP7, it was not at fault. What was happening was that I was returning a blobstore blob from GAE that did not exist. The GAE logs show this as a 200, but with a 0kb reply size, which is what eventually tipped me off. Would be nice if the logs showed this as the 500 that is in reality being returned.