The repository at http://src.seereason.com/o2bug contains a package that causes GHC-7.8.1 (rc2?) to exhaust 16GB of RAM and die when the -O2 flag is turned on.  I haven't been able to simplify it very much, almost any change to JSON.Render causes it to start working properly.  I did get it to fail using only packages available in hackage.  Just thought I should let folks know about this issue.  Let me know if it fails to fail.