I just had left work with a potential big issue left open (big defined as - "we may need to re-write the entire UI layer"), I was very pissed off, to say the least. I had a problem with the ASP.Net Session not working in Ajax call back. I just made a significant architecture change that require that I would store some information in the ASP.Net session, and I didn't feel like having to either re-write it or the UI layer.
Googling wasn't really helpful, which only made me angier. At that point, I got sick of it and went home, thinking very bad thoughts about some @#$#@%%. On the ride home, I suddenly had the answer. The issue was only happening when I made a CascadingDropDown call to the server, everything else was behaving normally. CascadingDropDown is using a WebService to get the data from the server. Now I can't think of a good reason that a Web Service should have an ASP.Net session by default.
The moment I knew what the underlying problem was, it was seconds to find what the solution was, until then, it was nearly impossible.