The small_oplog scons target (startMongodSmallOplog) runs two mongods in a master-slave pair, and the port 28000 used by the slave conflicts with the hard-coded port 29000 used by testing_only_commands.js (because of the http port opened 1000 higher than the base 28000 port). testing_only_commands.js should either use a different hard-coded port or, if possible, dynamically pick a port that does not conflict with the environment the test is running in.