SlowNightly tests were never run on Buildbot's Linux 64 DEBUG builder, so this is a newly discovered issue. It looks like memory.js takes around 440 minutes on MCI on a regular basis which, when added to the rest of the slowNight suite, adds up to more than 12 hours. This causes the suite to fail.

And example of a log output for this test taking too long:
http://buildlogs.mongodb.org/mci_0.9_linux_64_debug/builds/769/test/slow_nightly_tests_0/memory.js