We've experienced several crashes of our production mongod. The server is under reasonable load, with queries backed by indices and no long running queries. Here's one of the crashes:
Sat Feb 9 14:47:39 Backtrace:
0xaffd31 0x558bb9 0x559142 0x7fbf0f8f6500 0x7fbf0ecea60e 0x5c3be8 0x8f480b 0x901a0b 0x908769 0x909072 0x9097c7 0x8dda17 0x8de446 0x8e751a 0x8e8a78 0x8e8c3b 0x82bde4 0x6f1072 0x6e8d71 0x6ea700
/usr/bin/mongod(_ZN5mongo15printStackTraceERSo+0x21) [0xaffd31]
/usr/bin/mongod(_ZN5mongo10abruptQuitEi+0x399) [0x558bb9]
/usr/bin/mongod(_ZN5mongo24abruptQuitWithAddrSignalEiP7siginfoPv+0x262) [0x559142]
/lib64/libpthread.so.0(+0xf500) [0x7fbf0f8f6500]
/lib64/libc.so.6(+0x13d60e) [0x7fbf0ecea60e]
/usr/bin/mongod(ZN5mongo20compareElementValuesERKNS_11BSONElementES2+0x778) [0x5c3be8]
/usr/bin/mongod(_ZN5mongo10FieldRange9intersectERKS0_b+0x12b) [0x8f480b]
/usr/bin/mongod(_ZN5mongo13FieldRangeSet19intersectMatchFieldEPKcRKNS_11BSONElementEbb+0x5b) [0x901a0b]
/usr/bin/mongod(_ZN5mongo13FieldRangeSet16handleMatchFieldERKNS_11BSONElementEb+0x1b9) [0x908769]
/usr/bin/mongod(_ZN5mongo13FieldRangeSet4initEb+0xa2) [0x909072]
/usr/bin/mongod(_ZN5mongo13FieldRangeSetC1EPKcRKNS_7BSONObjEbb+0x147) [0x9097c7]
/usr/bin/mongod(ZN5mongo16MultiPlanScanner4initERKNS_7BSONObjES3_S3+0x87) [0x8dda17]
/usr/bin/mongod(ZN5mongo16MultiPlanScanner4makeEPKcRKNS_7BSONObjES5_RKN5boost10shared_ptrIKNS_11ParsedQueryEEES5_NS_18QueryPlanGenerator18RecordedPlanPolicyES5_S5+0x76) [0x8de446]
/usr/bin/mongod(_ZN5mongo15CursorGenerator19setMultiPlanScannerEv+0xea) [0x8e751a]
/usr/bin/mongod(_ZN5mongo15CursorGenerator8generateEv+0x98) [0x8e8a78]
/usr/bin/mongod(_ZN5mongo25NamespaceDetailsTransient9getCursorEPKcRKNS_7BSONObjES5_RKNS_24QueryPlanSelectionPolicyEPbRKN5boost10shared_ptrIKNS_11ParsedQueryEEEbPNS_16QueryPlanSummaryE+0x4b) [0x8e8c3b]
/usr/bin/mongod(_ZN5mongo8runCountEPKcRKNS_7BSONObjERSsRi+0x494) [0x82bde4]
/usr/bin/mongod(_ZN5mongo8CmdCount3runERKSsRNS_7BSONObjEiRSsRNS_14BSONObjBuilderEb+0x62) [0x6f1072]
/usr/bin/mongod(_ZN5mongo12_execCommandEPNS_7CommandERKSsRNS_7BSONObjEiRNS_14BSONObjBuilderEb+0x51) [0x6e8d71]
/usr/bin/mongod(_ZN5mongo11execCommandEPNS_7CommandERNS_6ClientEiPKcRNS_7BSONObjERNS_14BSONObjBuilderEb+0xe70) [0x6ea700]

In case it's relevant, we have 3 replication instances: a primary, a secondary, and an arbiter, all within the same ec2 zone.