I suspect this is a bug in glibc mcheck's pthread support, since ElectricFence with EF_PROTECT_BELOW set will detect 100% of all memory underruns, and no such error occurs there.
Also this report (which it seems the original is no longer accessible) implies the same problem with mcheck
http://www.google.com/search?q=cache:dAqJIBQSNRQJ:groups.google.com/group/gn...