Error MSB6006: “mt.exe” exited with code 31

Question

I ran into this problem today while migrating a VS2008 solution to VS2010. The problem occurred in either of the following scenarios:

1. Rebuild Solution

Answer 1

WARNING: This exact error can also happen as a result of having a Windows Explorer window open in the folder containing the *.exe which you are currently attempting to build.

For example: Explorer open in /.../MyProj/Debug/ while trying to build the Debug version of your code in Visual Studio.

I was angry with myself for ~10min before realizing this.