I think it's fair to say that if you've never had an “issue” debugging an ASP.NET application through Visual Studio .NET, you're either relatively new to ASP.NET or you're extremely lucky. You don't have to look much further than the microsoft.public.vsnet.debugging newsgroup to find a number of folks who have had problems debugging ASP.NET pages in Visual Studio .NET. For some common problems (and their solutions) see Roy Osherove's blog post The VS.NET 7 Debugger Doesn't Work. What Can I Do?
Yesterday I experienced another ASP.NET debugging problem that isn't addressed at Roy's post, or, to my knowledge, in the Microsoft KBs. I was working on a computer that had the ASP.NET 2.0 technology preview installed, which had tweaked IIS so that the ASP.NET-related file extensions - .aspx, .asmx, etc. - were mapped to the ASP.NET 2.0 ISAPI extension (WINDOWS\Microsoft.Net\Framework\v.1.2.xxxx\aspnet_isapi.exe), rather than the ASP.NET v1.1 assemblies (WINDOWS\Microsoft.Net\Framework\v.1.1.xxxx\aspnet_isapi.exe). With this setting, ASP.NET pages would work fine when visiting them without attaching the debugger, but attempting to attach the debugger resulted in an error. The solution was to reconfigure IIS to have the file extensions mapped to the v1.1 ISAPI extension.
This, sadly, was the last place I looked. I spent near an hour pouring through newsgroup posts trying a long list of different suggestions and techniques. The point is, if you have the Whidbey bits installed on your computer, and are having troubles debugging ASP.NET 1.x projects, consider checking the AppMappings in IIS first, before pulling your hair out! :-)