Project is permanently out of date

I've seen several cases with Visual Studio 2010 and later where my project would be permanently "out of date." I'd build the project, run it, and would immediately be told that my project was out of date. Solving this through trial and error is tedious at best. This problem commonly is caused by two different problems:

Missing file in project

The most common cause is a file that's in your project that you've deleted from the disk. These are easy to find in smaller projects by just looking through the project for a file with an X next to it. If you can't find the file causing the problem, enable detailed Diagnostic output in MSBuild. In Visual Studio, open Tools | Options, select Projects and Solutions | Build and Run, and set MSBuild project build output verbosity to Detailed.

Alternatively, here's a handy Python script that will examine a .vcxproj file and look for any missing files:
http://stackoverflow.com/questions/2762930/vs2010-always-thinks-project-is-out-of-date-but-nothing-has-changed

I've also seen this problem when I renamed a file. The old filename is still embedded in the compiler's temporary files, even though the name no longer appears in the project. The solution is to delete all intermediate files in the project and rebuild.

Project upgraded from earlier version of Visual Studio

The StackOverflow page above also details a bug where MSBuild says, "Forcing rebuild of all source files due to a change in the command line since the last build."

The most likely cause is if you used /MP in the Additional Options box in project Properties | Configuration Properties | C/C++ | Command Line. This was often done in Visual Studio 2005. My project started building properly after I fixed this (although it was tricky, because I had set /MP1 on certain files due to bugs in multiprocessor support in VC2005.) This tip came from https://social.msdn.microsoft.com/Forums/vstudio/en-US/aca632fa-e1e0-4511-aa03-a309ae547a5b/how-to-see-the-command-line-forcing-rebuild-of-all-source-files-due-to-a-change-in-the-command?forum=msbuild.

The next problem I saw in the MSBuild log was the use of an old PDB filename. I was using Visual Studio 2013, which uses vc120.pdb. However, some of my project files used the name vc80.pdb, which was the wrong name and so was never found. In the property page for the project, set it to look something like this: (the part in bold is what you see after you set the value for the first time. The next time you reopen the page, it will show you the value that the compiler will use.)

Unhelpful recommendations

After enabling Detailed output for MSBuild, I saw this error. It didn't appear to affect the problem, but it should probably be fixed to prevent confusion in the future:
Project file contains ToolsVersion="4.0". This toolset may be unknown or missing, in which case you may be able to resolve this by installing the appropriate version of MSBuild, or the build may have been forced to a particular ToolsVersion for policy reasons. Treating the project as if it had ToolsVersion="12.0". For more information, please see http://go.microsoft.com/fwlink/?LinkId=293424.

Another suggestion which didn't work was that intermediate directories use a relative path. but I use an absolute path in my project and it works fine:
https://social.msdn.microsoft.com/Forums/vstudio/en-US/36df2f40-f6d4-4123-8261-88394790f29e/project-always-needs-complete-rebuild-in-vs-2010?forum=vcgeneral