First reported by Kazu Yamamoto in http://www.haskell.org/pipermail/ghc-devs/2013-May/001346.html

On OSX x86 with 7.7.20130530:

$ ghc --make Main -O2 -Wall
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...
$ ./Main
Winner (E {_key = 156, prio = 9.443531728518195e-3}) (LLoser [...]
Before atMost
[E {_key = 2, prio = 6.2447874501499356e-2},[...]prio = 0.35312841343485957}Segmentation fault: 11

It randomly fails with either "Segmentation fault: 11" or "Bus error: 10". Sometimes it succeeds.

With -O it apparently always succeeds.