This was a small issue i came across the other day, just thought would add a note about it like i usually do.
Problem
When running my tests in visual studio everything is fine and they all pass. In this particular case they were actually BizUnit tests where some of my steps use a proxy class which i have created and is in a seperate assembly. In my assembly where i have my tests i have just added a reference even though my code does not directly use the assembly i reference.
The problem is when i run my tests of the command line using mstest. I get an error that the proxy type can not be found and it turns out that the assembly containing it hasnt been copied to the test run folder where as when running them in the IDE it is copied.
Solution
The way i solved this was to add a deployment item to the test run config. This will ensure the proxy assembly is copied to the folder regardless if the tests are ran from the command line or the IDE
Hope this saves someone a few minutes.