This is definitely not obvious for me.
I did some performance tuning for our web app and cached some pages a while back. I did this:
<%@ OutputCache Location="Any" Duration="3600" VaryByParam="none"%>
I was going through the IIS logs as see that the page in question is still being requested a lot. Changing it to
<%@ OutputCache Location="Any" Duration="3600" VaryByParam="None"%>
does the trick. It seems implicit and I have definitely seen other people using the lower case version. But the attribute is CASE-SENSITIVE!!!