GHC concurrency runtime "breaks" every 497 (and a bit) days
The GHC concurrency model does all of its time calculations for delays in "ticks"
- a tick being fixed at 1/50th of a second. It performs all of these calculations in terms the number of these ticks since the Unix epoch (Jan 1970). Which is stored as an Int - unfortunately this (typically) 2^31 bits overflows every 497 (and a bit) days.
When it does overflow any threads on the timer queue will either a) run immediately or b) not be run for another 497 days - depending on which way the sign changes. Also any timer calculation that spans the wrapround (sometime in the preceeding 35 or so minutes) will not be dealt with correctly.
Although this doesn't happen often (the last one was around Sat Sep 30 18:32:51 UTC 2006 and the next one will not be until Sat Feb 9 21:00:44 UTC 2008) it nees to be resolved.
Take a look at the definition of getTicksofDay (in base/include/HsBase.h (non-windows) / base/cbits/Win32Utils.c) and getDelay (in base/GHC/Conc.lhs) to understand the details of the issue.