I think I might know what your problem is. You're accepting file uploads using handleMultipart, yes? Snap kills uploads that are going too slow, otherwise you would be vulnerable to slowloris (http://ha.ckers.org/slowloris/) DoS attacks. What's probably happening here is that you're doing slow work inside the "Iteratee IO a" handler you pass to that function, which makes Snap think the client is trickling bytes to you. If that's the case, either finish the iteratee more quickly and do the slow work back in the Snap handler (preferable), or disable the minimum upload rate guard (although that's not recommended on a server talking to the public internet.)
I tried adding a "setMinimumUploadRate 0" to my handleMultipart and doing the upload, and it's still getting killed. The uploads are pretty fast; a 150MB file takes around 10s. I really don't think the problem is with Snap, but pulling my code out of snap would be pretty painful. I have some pretty nasty crap going on with what I'm doing anyhow, with threads communicating asynchronously, sockets being held open between requests, that sort of horrid ugliness. I'm going to try to get rid of that, and then see if any bugs I had in there were causing the problem. Hopefully I'll be done with that by the end of the weekend, and I'll either have the problem fixed, or have a reasonable way to reproduce the errors. Thanks for the idea, anyhow. And, if setMinimumUploadRate 0 doesn't actually disable the rate limiter, I'd be happy to try something more correct.