Hi, 

I have the following test: https://gist.github.com/2694960
I run this under Selenium.WebDriver.2.21.0
Windows 7 , Firefox v11

The testing framework in use is mbunit - Gallio. 

It essentially starts multiple instances of Firefox and under different threads it
runs those two tests in parallel.

Each time one of the two tests fail to run. The stack-trace of the failure is as follows:

Stack trace of the exception's thrown follows(sometimes #1 and sometimes #2)

#1 https://gist.github.com/2694979
#2 https://gist.github.com/2694985