IIS 7 Error Pages taking over 500 Errors

I've been running into a problem with IIS returning error pages when my application is actually returning 500 errors along with custom content. IOW, IIS is taking over error display completely even if I send my own content for errors. This has some profound effects on error handling and on some custom handler implementations that relying on returning error information to the client. Thankfully there's a new ASP.NET 3.5 property that helps with this problem, although it's a rather obscure fix for this issue.