There appears to be a bug in the WebConnectionStream class in Mono.
Using the SharpBox library I have came across this issue on a few occassions when using the DownloadFile method when trying to download large files e.g. 10mb+. From what I can gather about this it is a race condition due to too many reads happening. A workaround which can be applied to the SharpBox library was to increase the default buffer size from 4k to 8k in the StreamHelper class.
I have attached a patch for MonoTouch.