I am running the ball drop example found here http://code.haskell.org/wxhaskell/samples/wx/BouncingBalls.hs
Building on mac os x 10.5.8, compiling with a recent version of the Haskell platform and creating the .app directory with the mac-osxapp script.
When the user clicks in he window a red ball is created and bounces in the window. Additional balls can only be created if the mouse is moved slightly.
Why this should be the case is not clear from the code. Seems to be a bug.