I was getting a BadImageFormatException when trying to load my test assembly. 

I looked at the framework assembly versions and updated the config file for both the Nunit-console.exe.config and Nunit-gui.exe.config file. 

<startup>

   <supportedRuntime version="v2.0.50215" /> <!-- I updated this and moved to the top of the list-->
   <supportedRuntime version="v1.1.4322" />
   <supportedRuntime version="v1.0.3705" />
 
   <requiredRuntime version="v1.0.3705" />

  </startup>