[Too]

I once had extremely odd errors being reported by my clients (phonehomes via window.onerror in the browser), things like syntax errors in my otherwise perfectly fine javascript and randomly corrupted data transfered with ajax. So i started investigating and came to the conclusion that someones ISP was trying to inject iframes with ads into any random text transfered over http, including javascript! Pure craziness, if i myself was affected i would change ISP on the spot.

This case was extreme but imagine as you say, a proprietary client and server that think they have implemented HTTP properly but they haven't. Maybe they assume all headers come in a specific order, the request size may not exceed X bytes, a hash of the request has been transfered over another channel, etc etc. Normally this is fine because they always only communicate with each other and they both always do exactly the same "mistake", but now the data essentially becomes corrupted. Can you really blame these applications for "not following spec", they were only designed to communicate with each other.