Meaning of "Unexpected Step" exceptions in ThreadT

I'm working on writing some thread tests using Racer, and have come accross a rather un-obvious exception. After running about 70 scenarios, the test fails with the following exception:
----> Typemock.Racer.Core.Exceptions.RacerException : Unexpected Step instead of : b6 (1, False) ID=2709

Following this is more interesting information which is meaningless to me, but could be useful to you guys.

My biggest question is, what does this error indicate? Is it pointing to a problem in my code that should be corrected? If so what kinds of things can cause this error?

Any information you can provide would be greatly appreciated. If you would like I can also include more details from the Racer output of the test run.