Recently, we 've started getting the following assertions in log file when trying to delete documents (not on every delete, it happens occasionally, about 1 in a million documents deleted):

Thu May 3 13:09:39 [conn567441] Assertion: 10334:Invalid BSONObj size: -286331154 (0xEEEEEEEE) first element: _id:

{ F: 23866263275 29652228, C: 2, i: 288230376151857194, u: 288230376151872074 }

0x584102 0x508521 0x93df93 0x88ca07 0x88e8e0 0xaa33f6 0x637407 0x7f627b6669ca 0x7f627ac1570d
/usr/bin/mongod(_ZN5mongo11msgassertedEiPKc+0x112) [0x584102]
/usr/bin/mongod(_ZNK5mongo7BSONObj14_assertInvalidEv+0x471) [0x508521]
/usr/bin/mongod(_ZN5mongo13deleteObjectsEPKcNS_7BSONObjEbbbPNS_11RemoveSaverE+0xc93) [0x93df93]
/usr/bin/mongod(_ZN5mongo14receivedDeleteERNS_7MessageERNS_5CurOpE+0x437) [0x88ca07]
/usr/bin/mongod(_ZN5mongo16assembleResponseERNS_7MessageERNS_10DbResponseERKNS_11HostAndPortE+0x1210) [0x88e8e0]
/usr/bin/mongod(_ZN5mongo16MyMessageHandler7processERNS_7MessageEPNS_21AbstractMessagingPortEPNS_9LastErrorE+0x76) [0xaa33f6]
/usr/bin/mongod(_ZN5mongo3pms9threadRunEPNS_13MessagingPortE+0x287) [0x637407]
/lib/libpthread.so.0(+0x69ca) [0x7f627b6669ca]
/lib/libc.so.6(clone+0x6d) [0x7f627ac1570d]

The record with the mentioned _id is not to be found in the collection (we are not sure if was deleted or wasn't there before in the first place).
This assertion looks intimidating and almost cries "data corruption" for us, the only thing that makes us feel safer is that complete daily DB dump (using mongodump) does not signal any errors.