Merge pull request #5054 from yaananth/patch-1

Add warning against using PreSendRequestHeaders

aspnet/aspnet/overview/web-development-best-practices/what-not-to-do-in-aspnet-and-what-to-do-instead.md

@@ -199,7 +199,9 @@ The following example shows how to pass an encoded URL as a query string paramet
 
 Recommendation: Do not use these events with managed modules. Instead, write a native IIS module to perform the required task. See [Creating Native-Code HTTP Modules](https://msdn.microsoft.com/en-us/library/ms693629.aspx).
 
-You can use the [PreSendRequestHeaders](https://msdn.microsoft.com/en-us/library/system.web.httpapplication.presendrequestheaders.aspx) and [PreSendRequestContext](https://msdn.microsoft.com/en-us/library/system.web.httpapplication.presendrequestcontent.aspx) events with native IIS modules, but do not use them with managed modules that implement IHttpModule. Setting these properties can cause issues with asynchronous requests.
+You can use the [PreSendRequestHeaders](https://msdn.microsoft.com/en-us/library/system.web.httpapplication.presendrequestheaders.aspx) and [PreSendRequestContext](https://msdn.microsoft.com/en-us/library/system.web.httpapplication.presendrequestcontent.aspx) events with native IIS modules.
+> [!WARNING]
+> Do not use `PreSendRequestHeaders` and PreSendRequestContext` with managed modules that implement `IHttpModule`. Setting these properties can cause issues with asynchronous requests. We have seen instances where the combination of Application Requested Routing(ARR) and websockets lead to access violation exceptions (iiscore!W3_CONTEXT_BASE::GetIsLastNotification+68 in iiscore.dll has caused an access violation exception (0xC0000005)) that lead to w3wp crashing.
 
 <a id="asyncevents"></a>