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