One anomalie we have encountered through our performance testing is that
SharePoint is setting an invalid HTTP header response in some cases. It
appears there is a typo in the "Expires" header. There's a missing "p"
. It only seems to happen on a 304 response but could have some adverse
effects on cached content. My guess is this is a custom header being
generated by SharePoint.

I installed MOSS 2007 on two separate boxes and this behavior is
consistent.

You can test this by doing a header watch when accessing
/Pages/Default.aspx

HTTP/1.x 304 NOT MODIFIED
Date: Fri, 09 Mar 2007 18:27:55 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
Exires: Thu, 22 Feb 2007 18:27:55 GMT
Cache-Control: private,max-age=0
Content-Length: 0