Long story short, I have an ASP.NET application I'm trying to debug and at some point, in very particular circumstances, the application will throw exceptions at a
"Cannot redirect after HTTP headers have been sent."
HttpApplication has an event
PreSendRequestHeaders which is called just when headers are writtne. Subscribe to this and log it or add a breakpoint.
HttpResponse has a internal property called
_headersWritten field in .NET 1.1). Since it's internal you can't access it directly, but you can through reflection. If this is only for internal debugging (i.e., not production code), then it's be fine to use reflection.
Check this method before/after all the page lifecylce events. Once you know which event is writing out the headers, add more
HeadersWritten checks to find out where they're getting written. Through progressive narrowing of checks to this property, you'll find it.