As is suggested elsewhere, I am using redirectMode = ResponseRewrite in my custom error configuration so my custom error page can access the exception info. This has worked like a charm for some time.
In adding some more "help the user recover from an error" type functionality, we need a piece of info that has been previously stored in Session. When implementing this, I found that the various avenues to Session end in null when redirectMode=ResponseRewrite, but they are all populated when redirectMode=ResponseRedirect (or isn't defined).
Anyone know why? It seems odd that we'd have to choose between having exception info (ResponseRewrite) or having Session (ResponseRedirect).
The MSDN article on Rich Custom Error handling tells me that Session is only available when the control passing method is Server.Transfer, which is what I assumed ResponseRewrite used under the hood. Evidently that isn't the case.