I ran a command-line repair on one server in 2 server+arbiter replica set to try to reclaim some disk space and got the following error:

[initandlisten] Assertion: 10446:mmap: can't map area of size 0 file: /data/db/_tmp/esort.1310694712.469162920//file.9
0x55f5aa 0x5ba9f8 0x7a5eb5 0x7a994e 0x776b40 0x77874d 0x77a0ae 0x77b4cf 0x726458 0x727dcd 0x728e0d 0x78349a 0x8aab98 0x8abad4 0x8acecc 0x8ada88 0x8b36bf 0x7f2daff0ac1d 0x4e10c9
mongod(_ZN5mongo11msgassertedEiPKc+0x12a) [0x55f5aa]
mongod(_ZN5mongo16MemoryMappedFile3mapEPKcRyi+0x438) [0x5ba9f8]
mongod(_ZN5mongo21BSONObjExternalSorter12FileIteratorC1ESs+0x45) [0x7a5eb5]
mongod(ZN5mongo21BSONObjExternalSorter8IteratorC1EPS0+0x13e) [0x7a994e]
mongod(_ZN5mongo14fastBuildIndexEPKcPNS_16NamespaceDetailsERNS_12IndexDetailsEi+0xc70) [0x776b40]
mongod() [0x77874d]
mongod(_ZN5mongo11DataFileMgr6insertEPKcPKvibRKNS_11BSONElementEb+0xf4e) [0x77a0ae]
mongod(_ZN5mongo11DataFileMgr16insertWithObjModEPKcRNS_7BSONObjEb+0x5f) [0x77b4cf]
mongod(_ZN5mongo6Cloner4copyEPKcS2_bbbbNS_5QueryE+0x3a8) [0x726458]
mongod(_ZN5mongo6Cloner2goEPKcRSsRKSsbbbb+0x10cd) [0x727dcd]
mongod(_ZN5mongo9cloneFromEPKcRSsRKSsbbbb+0x3d) [0x728e0d]
mongod(_ZN5mongo14repairDatabaseESsRSsbb+0x46a) [0x78349a]
mongod(_ZN5mongo11doDBUpgradeERKSsSsPNS_14DataFileHeaderE+0x68) [0x8aab98]
mongod() [0x8abad4]
mongod(_ZN5mongo14_initAndListenEiPKc+0x41c) [0x8acecc]
mongod(_ZN5mongo13initAndListenEiPKc+0x18) [0x8ada88]
mongod(main+0x5acf) [0x8b36bf]
/lib64/libc.so.6(__libc_start_main+0xfd) [0x7f2daff0ac1d]
mongod(__gxx_personality_v0+0x3a1) [0x4e10c9]
Fri Jul 15 01:54:23 [initandlisten] exception in initAndListen std::exception: mmap: can't map area of size 0 file: /data/db/_tmp/esort.1310694712.469162920//file.9, terminating

The repair was successful on the other server.