Here is what my Event Log says:

Event code: 3006
Event message: A parser error has occurred.
Event time: 7/4/2007 5:39:16 PM
Event time (UTC): 7/4/2007 9:39:16 PM
Event ID: 8cbd67cf09854f4095d0321402a845b8
Event sequence: 4
Event occurrence: 1
Event detail code: 0

Application information:
   Application domain: 99dad5e2-11-128280587552343750
   Trust level: Full
   Application Virtual Path: /
   Application Path: D:\Company\Projects\Queue Web Service\
   Machine name: Computer

Process information:
   Process ID: 2024
   Process name: WebDev.WebServer.exe
   Account name: Computer\XXXX

Exception information:
   Exception type: HttpParseException
   Exception message: Could not create type Company.NA.QueueAPI'.

Request information:
   Request URL: http://localhost:1884/QueueAPI.asmx?disco
   Request path: /QueueAPI.asmx
   User host address: 127.0.0.1
   User: Computer\XXXX
   Is authenticated: True
   Authentication Type: NTLM
   Thread account name: Computer\XXXX

Thread information:
   Thread ID: 4
   Thread account name: Computer\XXXX
   Is impersonating: False
   Stack trace:    at System.Web.UI.SimpleWebHandlerParser.GetType
(String typeName)
  at System.Web.UI.SimpleWebHandlerParser.GetTypeToCache(Assembly
builtAssembly)
  at System.Web.Compilation.SimpleHandlerBuildProvider.GetGeneratedType
(CompilerResults results)
  at System.Web.Compilation.BuildProvider.CreateBuildResult
(CompilerResults results)
  at System.Web.Compilation.BuildProvider.GetBuildResult
(CompilerResults results)
  at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath
virtualPath)
  at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal
(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp,
Boolean allowBuildInPrecompile)
  at
System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert
(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean
allowCrossApp, Boolean allowBuildInPrecompile)
  at System.Web.Compilation.BuildManager.GetVPathBuildResult
(HttpContext context, VirtualPath virtualPath)
  at System.Web.UI.WebServiceParser.GetCompiledType(String inputFile,
HttpContext context)
  at System.Web.Services.Protocols.WebServiceHandlerFactory.GetHandler
(HttpContext context, String verb, String url, String filePath)
  at System.Web.HttpApplication.MapHttpHandler(HttpContext context,
String requestType, VirtualPath path, String pathTranslated, Boolean
useAppConfig)
  at
System.Web.HttpApplication.MapHandlerExecutionStep.System.Web.HttpApplic
ation.IExecutionStep.Execute()
  at System.Web.HttpApplication.ExecuteStep(IExecutionStep step,
Boolean& completedSynchronously)


Custom event details:

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Problem solved?

Turns out WSE was intercepting some error messages ... I think one of my
objects has a read-only property (set is set as private). It seems if you
have an input object with such a property... ASP.NET chokes. If you have
WSE enabled, it hides the error messages :-(