Hey! I have some code that is not behaving the way I thought it should. The gist of it is sleeper = mask_ $ forkIOWithUnmask $ \restore -> forever $ restore sleep `catch` throwBack throwBack (Ping tid) = myThreadId >>= throwTo tid . Pong throwBack (Pong tid) = myThreadId >>= throwTo tid . Ping Since (a) throwBack is executed on a masked state, (b) myThreadId is uninterruptible, and (c) throwTo is uninterruptible, my understanding is that the sleeper thread should catch all PingPong exceptions and never let any one of them through. However, that's not what I am seeing. Running with -threaded and at least -N2 I can reliably get sleeper to be killed. Please see the complete code at: https://gist.github.com/meteficha/5390079 The expected result is 100 Pongs and 0 Pings. Running with -N1 that's what I get. Running with -N2 I get a few Pongs and then 1 Ping. Running with -N3 or more usually gives me just 1 Ping and not a single Pong. I'm testing with GHC 7.4.1. I've tried to download 7.7 but---either because of haskell.org or my ISP---I'm not being able to download it. (Note that this problem is not academic as I've been bitten by it on my production code.) Thanks! =D -- Felipe.