rs.stepDown() on any PRIMARY being monitored using mongostat:

mongodb-linux-x86_64-1.7.5/bin/mongostat -h rs1a:27018,rs1b:27018,rs1c:27018,rs1d:27018,rs2a:27018,rs2b:27018,rs2c:27018,rs2d:27018,rs3a:27018,rs3b:27018,rs3c:27018,rs3d:27018
...
rs1a:27018 *0 *0 *2 *0 0 1|0 0 408g 409g 1.45g 0 0 0 0|0 0|0 62b 1k 53 set1 SEC 13:16:07
rs1b:27018 0 2 0 0 0 11 0 414g 435g 3.27g 0 0 0 0|0 2|0 1k 13k 2083 set1 M 13:16:07
rs1c:27018 *0 *0 *2 *0 0 5|0 0 410g 411g 3.32g 0 0 0 0|0 0|0 548b 1k 18 set1 SEC 13:16:07
rs1d:27018 *351 *0 *61 *67 0 12|0 0 408g 409g 3.11g 145 97.8 0 0|0 0|0 917b 5k 23 set1 SEC 13:16:07
rs2a:27018 0 0 0 0 1 25 0 350g 373g 588m 0 0 0 0|0 1|0 2k 5k 2308 set2 M 13:16:07
rs2b:27018 *0 1 *0 *0 0 3|0 0 350g 352g 1.26g 0 0 0 0|0 0|0 374b 2k 139 set2 SEC 13:16:07
rs2c:27018 socket exception
rs2d:27018 *0 *0 *0 *98 0 4|0 0 336g 336g 1.67g 3 3.7 0 0|0 0|0 428b 1k 13 set2 SEC 13:16:07
rs3a:27018 socket exception
rs3b:27018 0 7 0 0 1 18 0 86.1g 109g 1.77g 17 18.7 0 9|857 9|861 91k 94k 2326 set3 M 13:16:07
rs3c:27018 *0 *0 *1 *0 0 1|0 0 80.1g 80.5g 1.19g 2 0.2 0 0|0 0|0 62b 1k 14 set3 SEC 13:16:07
rs3d:27018 *0 *0 *0 *0 0 2|0 0 80.1g 80.6g 1.65g 0 0 0 0|0 0|0 182b 1k 19 set3 SEC 13:16:07
Sun Jan 30 13:16:08 Assertion failure isABSONObj() db/../bson/bson-inl.h 41
0x54e79e 0x55ffc1 0x4ec6be 0x8ad45d 0x8b3d6b 0x8b6653 0x8a4754 0x8a6162 0x34f461d994 0x4def49
mongodb-linux-x86_64-1.7.5/bin/mongostat(_ZN5mongo12sayDbContextEPKc+0xae) [0x54e79e]
mongodb-linux-x86_64-1.7.5/bin/mongostat(_ZN5mongo8assertedEPKcS1_j+0x111) [0x55ffc1]
mongodb-linux-x86_64-1.7.5/bin/mongostat(_ZNK5mongo11BSONElement14embeddedObjectEv+0x3e) [0x4ec6be]
mongodb-linux-x86_64-1.7.5/bin/mongostat(ZN5mongo4Stat5doRowERKNS_7BSONObjES3+0xa2d) [0x8ad45d]
mongodb-linux-x86_64-1.7.5/bin/mongostat(_ZN5mongo4Stat7runManyEv+0xa2b) [0x8b3d6b]
mongodb-linux-x86_64-1.7.5/bin/mongostat(_ZN5mongo4Stat3runEv+0x253) [0x8b6653]
mongodb-linux-x86_64-1.7.5/bin/mongostat(_ZN5mongo4Tool4mainEiPPc+0x14d4) [0x8a4754]
mongodb-linux-x86_64-1.7.5/bin/mongostat(main+0x32) [0x8a6162]
/lib64/libc.so.6(__libc_start_main+0xf4) [0x34f461d994]
mongodb-linux-x86_64-1.7.5/bin/mongostat(__gxx_personality_v0+0x3a9) [0x4def49]
assertion: 0 assertion db/../bson/bson-inl.h:41