Here is the log dump right before the crash:

Thu Mar 17 02:53:02 [conn878465] CMD: drop mydb.tmp.mr.mapreduce_1300330381_18145
Thu Mar 17 02:53:02 [conn878465] CMD: drop mydb.tmp.mr.mapreduce_1300330382_18146
Thu Mar 17 02:53:02 [conn865568] building new index on

for mydb.tmp.mr.mapreduce_1300330382_18146
Thu Mar 17 02:53:02 [conn865568] done for 0 records 0secs
Thu Mar 17 02:53:02 Got signal: 11 (Segmentation fault).

Thu Mar 17 02:53:02 Backtrace:
0x824629 0x7fcb40d20af0 0x64124f 0x64227a 0x7643f2 0x797117 0x798538 0x5fb7e5 0x60029f 0x7074ba 0x70aaf6 0x82691b 0x83a4b0 0x7fcb418249ca 0x7fcb40dd370d
/usr/local/bin/mongod(_ZN5mongo10abruptQuitEi+0x399) [0x824629]
/lib/libc.so.6(+0x33af0) [0x7fcb40d20af0]
/usr/local/bin/mongod(_ZN5mongo12ClientCursor17setLastLoc_inlockENS_7DiskLocE+0x2f) [0x64124f]
/usr/local/bin/mongod(_ZN5mongo12ClientCursorD1Ev+0x4a) [0x64227a]
/usr/local/bin/mongod(_ZN5mongo2mr16MapReduceCommand3runERKSsRNS_7BSONObjERSsRNS_14BSONObjBuilderEb+0x1f42) [0x7643f2]
/usr/local/bin/mongod(_ZN5mongo11execCommandEPNS_7CommandERNS_6ClientEiPKcRNS_7BSONObjERNS_14BSONObjBuilderEb+0x597) [0x797117]
/usr/local/bin/mongod(_ZN5mongo12_runCommandsEPKcRNS_7BSONObjERNS_10BufBuilderERNS_14BSONObjBuilderEbi+0x798) [0x798538]
/usr/local/bin/mongod(_ZN5mongo11runCommandsEPKcRNS_7BSONObjERNS_5CurOpERNS_10BufBuilderERNS_14BSONObjBuilderEbi+0x35) [0x5fb7e5]
/usr/local/bin/mongod(ZN5mongo8runQueryERNS_7MessageERNS_12QueryMessageERNS_5CurOpES1+0x1bbf) [0x60029f]
/usr/local/bin/mongod() [0x7074ba]
/usr/local/bin/mongod(_ZN5mongo16assembleResponseERNS_7MessageERNS_10DbResponseERKNS_8SockAddrE+0x14d6) [0x70aaf6]
/usr/local/bin/mongod(_ZN5mongo10connThreadEPNS_13MessagingPortE+0x30b) [0x82691b]
/usr/local/bin/mongod(thread_proxy+0x80) [0x83a4b0]
/lib/libpthread.so.0(+0x69ca) [0x7fcb418249ca]
/lib/libc.so.6(clone+0x6d) [0x7fcb40dd370d]

Thu Mar 17 02:53:02 dbexit:

Thu Mar 17 02:53:02 [conn865568] shutdown: going to close listening sockets...
Thu Mar 17 02:53:02 [conn865568] closing listening socket: 11
Thu Mar 17 02:53:02 [conn865568] closing listening socket: 12
Thu Mar 17 02:53:02 [conn865568] closing listening socket: 13
Thu Mar 17 02:53:02 [conn865568] closing listening socket: 14
Thu Mar 17 02:53:02 [conn865568] shutdown: going to flush oplog...
Thu Mar 17 02:53:02 [conn865568] shutdown: going to close sockets...
Thu Mar 17 02:53:02 [conn865568] shutdown: waiting for fs preallocator...
Thu Mar 17 02:53:02 [conn865568] shutdown: closing all files...
Thu Mar 17 02:53:02 [conn2] end connection 127.0.0.1:45122
Thu Mar 17 02:53:02 [conn878461] end connection 10.124.251.55:49654
Thu Mar 17 02:53:02 [conn17] end connection 10.122.123.37:46455
Thu Mar 17 02:53:03 closeAllFiles() finished

Thu Mar 17 02:53:03 [conn865568] shutdown: removing fs lock...
Thu Mar 17 02:53:03 dbexit: really exiting now
Thu Mar 17 02:53:03 Got signal: 6 (Aborted).

Thu Mar 17 02:53:03 Backtrace:
0x824629 0x7fcb40d20af0 0x7fcb40d20a75 0x7fcb40d245c0 0x7fcb40d19941 0x641158 0x7fcb40d26262 0x702497 0x824773 0x7fcb40d20af0 0x64124f 0x64227a 0x7643f2 0x797117 0x798538 0x5fb7e5 0x60029f 0x7074ba 0x70aaf6 0x82691b
/usr/local/bin/mongod(_ZN5mongo10abruptQuitEi+0x399) [0x824629]
/lib/libc.so.6(+0x33af0) [0x7fcb40d20af0]
/lib/libc.so.6(gsignal+0x35) [0x7fcb40d20a75]
/lib/libc.so.6(abort+0x180) [0x7fcb40d245c0]
/lib/libc.so.6(__assert_fail+0xf1) [0x7fcb40d19941]
/usr/local/bin/mongod() [0x641158]
/lib/libc.so.6(exit+0xe2) [0x7fcb40d26262]
/usr/local/bin/mongod(_ZN5mongo6dbexitENS_8ExitCodeEPKc+0x147) [0x702497]
/usr/local/bin/mongod(_ZN5mongo10abruptQuitEi+0x4e3) [0x824773]
/lib/libc.so.6(+0x33af0) [0x7fcb40d20af0]
/usr/local/bin/mongod(_ZN5mongo12ClientCursor17setLastLoc_inlockENS_7DiskLocE+0x2f) [0x64124f]
/usr/local/bin/mongod(_ZN5mongo12ClientCursorD1Ev+0x4a) [0x64227a]
/usr/local/bin/mongod(_ZN5mongo2mr16MapReduceCommand3runERKSsRNS_7BSONObjERSsRNS_14BSONObjBuilderEb+0x1f42) [0x7643f2]
/usr/local/bin/mongod(_ZN5mongo11execCommandEPNS_7CommandERNS_6ClientEiPKcRNS_7BSONObjERNS_14BSONObjBuilderEb+0x597) [0x797117]
/usr/local/bin/mongod(_ZN5mongo12_runCommandsEPKcRNS_7BSONObjERNS_10BufBuilderERNS_14BSONObjBuilderEbi+0x798) [0x798538]
/usr/local/bin/mongod(_ZN5mongo11runCommandsEPKcRNS_7BSONObjERNS_5CurOpERNS_10BufBuilderERNS_14BSONObjBuilderEbi+0x35) [0x5fb7e5]
/usr/local/bin/mongod(ZN5mongo8runQueryERNS_7MessageERNS_12QueryMessageERNS_5CurOpES1+0x1bbf) [0x60029f]
/usr/local/bin/mongod() [0x7074ba]
/usr/local/bin/mongod(_ZN5mongo16assembleResponseERNS_7MessageERNS_10DbResponseERKNS_8SockAddrE+0x14d6) [0x70aaf6]
/usr/local/bin/mongod(_ZN5mongo10connThreadEPNS_13MessagingPortE+0x30b) [0x82691b]

Thu Mar 17 02:53:03 dbexit: ; exiting immediately

Thu Mar 17 02:53:04 ERROR: Client::~Client _context should be null but is not; client:conn