OK, here's the info I was thinking about. Shouldn't store an STA COM component in Session state since it will pin requests to that single thread that created the object.

A quote from the article:
You should never store any STA COM component in a shared resource where it can be accessed by threads other than the one that constructed it. This includes the cache and session state, for example.

Will have to come up with a way to prove this to myself...
Spit out thread ID in the page, and hammer a bunch of requests at it?

ASP.NET Optimization