URLLoader receives IOError

we use actionScript 3 to build a Flash video ad player. When we use URLLoader.as load(), we sometimes receive IOError.

Questions:

1. under what circumstances will an IOError happen?

2. When the player receives IOErrors, 80% of time the server didn't receive the request. It makes sense. However, there is another 20% of time, server did actually receive the requests. Why would URLLoader receive an IOError when the server actually receives and logs the request?

3. is there a deterministic way to tell if the request fails to hit the server?

Many thanks.