Application_Error not firing when identity impersonate="true"

[Brian]

Greetings group! I've got a weird one.

I have an ASP.NET 1.1 application that has been running on a
dual-processor Windows 2000/IIS 5 server for a couple of years.
Global.asax has an Application_Error() function that writes an entry to
the event log. Everything works great.

We're migrating to Windows 2003 on a new server, so I built up a
virtual PC (MS Virtual Server) Win2k3/IIS6 (single processor) box for
testing and everything works great there too.

So now I'm deploying to the real server - a dual-processor Windows
2003/IIS6 box and my Application_Error method is not getting called. So
I stripped everything out and here's what I have:

global.asax:
Application_Start() announces the app is starting with an event log
entry.
Application_End() does the same for the app stopping.
Application_Error() writes a simple "hello"

I have an index.aspx that simply generates an divide-by-0 error.

I also have web.config With NOTHING else but identity impersonate =
"false"

Nothing else - no other pages, folders, etc.

This works great - I see all 3 log entries. As soon as I turn
impersonation on (true), Application_Error stops getting called. I see
the 'starting' and 'stopping' log entries, so I know _Start() and
_End() are getting called and that I have the right permissions to
write to the event log, but no error entry. I do see the 'you got an
error' web page from ASP.NET.

I do have a user configured for this site for SQL Server access, that's
why I'm impersonating. But I haven't even set that up for this test
site. Out-of-the-box identity impersonation with anonymous access and
windows authentication should impersonate as IUSER_machine.

What might be happening here? Something to do with multiple processors?

Thanks for reading!

-Brian