First instance on the following commit: https://github.com/mongodb/mongo/commit/dcd569ac549275cd3b81642325f63001cb33cd64 seems to have caused some wonkiness with rollback4.js, e.g.
http://mci.10gen.com/ui/task/mongodb_mongo_master_cxx11_ubuntu1204_64_dcd569ac549275cd3b81642325f63001cb33cd64_13_11_06_15_00_08_slow_weekly_tests_ubuntu1204
http://mci.10gen.com/ui/task/mongodb_mongo_master_linux_64_1cf65f1b1f63784033ea21d7cd656c6ee961e6f1_13_11_06_15_52_16_slow_weekly_tests_linux_64
All of them have the same assertion failure:
assert.soon failed: function ()

{ return B.isMaster().ismaster; }, msg:undefined
Error: Printing Stack Trace
at printStackTrace (src/mongo/shell/utils.js:38:15)
at doassert (src/mongo/shell/assert.js:6:5)
at Function.assert.soon (src/mongo/shell/assert.js:174:60)
at /data/mci/git@github.commongodb/mongo.git/master/jstests/slowWeekly/rollback4.js:42:8
2013-11-06T13:16:13.071-0500 assert.soon failed: function () { return B.isMaster().ismaster; }

, msg:undefined at src/mongo/shell/assert.js:7