I got the following dialog box from Visual Studio on debugging a Web Application:

Microsoft Visual Studio
The following module was built either with optimizations enabled or without debug information:

C:UsersMyUserAppDataLocalTempTemporary ASP.NET Files1b3abf23b2c10ebaassemblydl3ba72d55bc2ecd6a3_6a4bce01MyDomain.MyProject.dll

To debug this module, change its project build configuration to Debug mode. To suppress this message, disable the 'Warn if no user code on launch' debugger option.

Having clicked OK on the dialog everything ran as normal though the offending DLL was highlighted in the Modules window (Debug > Windows > Modules). In fact the web application bin directory did contain this DLL without any corresponding PDB file.

This DLL had been previously used by the application but had been removed from the dependencies. However, both the Rebuild and the Clean actions left it because it was no longer a dependency. Once the bin directory had been cleaned out and the application rebuilt the DLL no longer appeared and the problem went away.

Using: Visual Studio 2012;