I ran into so many bizarre problems with obscure web containers that I was forced to do all sorts of HTTP traces, repeatedly read the specs, etc. A memorable problem was a version of Chilisoft truncating SiteMinder's ginormous SMSESSION cookie. Identifying that defect took some work. Sun was awesome to deal with though - I remember them getting us a patch in 2 days.
Anyway, its an older book, but I'm reading HTTP: The Definitive Guide. I am surprised at how much I still don't know about HTTP. Anyway, I really like the book - it makes me like HTTP even more.