What happens to REST requests when all threads are busy

HFCDev

I normally wouldn't ask this question, but we're seeing something to suggest that REST requests may not behave the same as regular request handlers, or even the Web Services API.  I'm used to the notion that if all threads are consumed, that the next request would be blocked until a thread becomes available. What we have anecdotally seen is that if a client executes multiple requests asynchronously, that some will succeed and some will fail. If the same operation is repeated, all the requests suceed.  The failure is a 500 server error.  I guess my first question is, does REST API request handler block until it gets an available thread as do regular request handlers?  If so, then the 500 error might not even be coming from Content Server but rather IIS.
Thanks in advance
-Hugh