System.MissingMethodException while UnitTesting in VS 2008

I've run across a strange situation where I have recently modified a WCF service and service contract adding new functionality. All I have done so far is stub in a new method with the appropriate updates to the service, the service contract, & the proxy (which is hand coded). When I try to run my existing unit tests, I am now getting the following error:

Class Initialization method <namespace>.<classname>_UnitTests.MyClassInitialize threw exception. System.MissingMethodException: System.MissingMethodException: Method not found: '<namespace>.<returntype> <namespace>.<contractinterface>.<methodname>()'.

Ironically, the method does exist and if I run the unit test in debug mode, the exception does not occur. If I run all the tests in debug mode, they all pass without a problem (expected result)...but if I just run the tests without being in debug mode, they fail with the MissingMethodException. IF I run the tests in debug mode, get them to all pass, then run them without restarting visual studio and being in debug mode, they pass. This is obviously some kind of a caching issue.

As it turns out, the issue appeared to be related to the *.testrunconfig file. Initially, code coverage was enabled. I disabled code coverage then deleted the testrunconfig file. I then cleaned the solution, closed visual studio, re-opened visual studio, & rebuilt the solution. The unit tests then began passing