GHCi, version 6.4.1, crashed with the following error: ghc-6.4.1: panic! (the `impossible' happened, GHC version 6.4.1): thread blocked indefinitely Please report it as a compiler bug to glasgow-haskell-bugs@haskell.org, or http://sourceforge.net/projects/ghc/. I was calling a test function which was forking a Thread which was blocking in an 'atomically' waiting on a TChan. If I ran the test function a second time, while the Thread from the first run is presumably still hanging around, I would receive the above error/crash. If I reloaded the source in GHCi after running the test function a single time GHCi would give an error message (but not crash): *** Exception: thread blocked indefinitely Most of the time after the error message there would be no modules loaded. I could load again, and repeat the fun. _Very_ occasionaly the modules would remain loaded and I could repeat the cycle without having to reload the modules. Since fixing my test function so the forked Thread dies at the desired time (ie now it won't be hanging around blocked on the TChan) I haven't observed the failure. My email: kgrapone@gmail.com