I was running (32-bit) Mongo when it suddenly went down. I looked in /var/log/mongo/mongod.log and saw the following:

============================================================================================================================================================
Tue Aug 2 05:07:01 Invalid access at address: 0x22917

Tue Aug 2 05:07:01 Got signal: 11 (Segmentation fault).

Tue Aug 2 05:07:01 Backtrace:
0x8523182 0x852388d 0x41640c 0xb6662a 0x829e5d9 0x82a10e4 0x83f1c08 0x8401454 0x8402268 0x8402539 0x840332d 0x828dcf5 0x83b0c47 0x83b6bc1 0x8526733 0x81f7c58 0x853a7ed 0x2399c9 0xbca6ee
/usr/bin/mongod(_ZN5mongo10abruptQuitEi+0x3c2) [0x8523182]
/usr/bin/mongod(_ZN5mongo24abruptQuitWithAddrSignalEiP7siginfoPv+0x25d) [0x852388d]
[0x41640c]
/lib/libc.so.6(memcpy+0x5a) [0xb6662a]
/usr/bin/mongod(_ZN5mongo22fillQueryResultFromObjERNS_10BufBuilderEPNS_10ProjectionERKNS_7BSONObjEPNS_7DiskLocE+0xa49) [0x829e5d9]
/usr/bin/mongod(_ZN5mongo11UserQueryOp4nextEv+0xc74) [0x82a10e4]
/usr/bin/mongod(_ZN5mongo12QueryPlanSet6Runner6nextOpERNS_7QueryOpE+0x38) [0x83f1c08]
/usr/bin/mongod(_ZN5mongo12QueryPlanSet6Runner3runEv+0x8c4) [0x8401454]
/usr/bin/mongod(_ZN5mongo12QueryPlanSet5runOpERNS_7QueryOpE+0x2e8) [0x8402268]
/usr/bin/mongod(_ZN5mongo16MultiPlanScanner9runOpOnceERNS_7QueryOpE+0x59) [0x8402539]
/usr/bin/mongod(_ZN5mongo16MultiPlanScanner5runOpERNS_7QueryOpE+0x2d) [0x840332d]
/usr/bin/mongod(ZN5mongo8runQueryERNS_7MessageERNS_12QueryMessageERNS_5CurOpES1+0x1575) [0x828dcf5]
/usr/bin/mongod() [0x83b0c47]
/usr/bin/mongod(_ZN5mongo16assembleResponseERNS_7MessageERNS_10DbResponseERKNS_8SockAddrE+0x711) [0x83b6bc1]
/usr/bin/mongod(_ZN5mongo10connThreadEPNS_13MessagingPortE+0x273) [0x8526733]
/usr/bin/mongod(_ZN5boost6detail11thread_dataINS_3_bi6bind_tIvPFvPN5mongo13MessagingPortEENS2_5list1INS2_5valueIS6_EEEEEEE3runEv+0x18) [0x81f7c58]
/usr/bin/mongod(thread_proxy+0x7d) [0x853a7ed]
/lib/libpthread.so.0(+0x69c9) [0x2399c9]
/lib/libc.so.6(clone+0x5e) [0xbca6ee]

Tue Aug 2 05:07:01 dbexit:
Tue Aug 2 05:07:01 [conn1949] shutdown: going to close listening sockets...
Tue Aug 2 05:07:01 [conn1949] closing listening socket: 5
Tue Aug 2 05:07:01 [conn1949] closing listening socket: 6
Tue Aug 2 05:07:01 [conn1949] closing listening socket: 7
============================================================================================================================================================

As far as I know, nothing fishy was happening client-side, just the usual hourly dump of data into Mongo.