c = db.c;
c.drop();
c.ensureIndex( { pos : "geoHaystack", type : 1 }, { bucketSize : 1 } );
printjson( c.find( { pos:4 } ).explain( true ) );

Wed Feb 20 16:42:36.140 [conn2]  test.c Assertion failure 0 src/mongo/db/geo/haystack.cpp 178
0x1064cbc55 0x10648f16b 0x10646d829 0x105f75e1a 0x10617c21e 0x106198f09 0x10619b8f7 0x10619bc34 0x10607d7bd 0x106082a3d 0x105fdd94b 0x105fe11d4 0x105be52bf 0x1064b0910 0x1064aeee1 0x1064aef46 0x1064aef81 0x106541439 0x7fff8514f8bf 0x7fff85152b75 
 0   mongod                              0x00000001064cbc55 _ZN5mongo15printStackTraceERSo + 37
 1   mongod                              0x000000010648f16b _ZN5mongo10logContextEPKc + 123
 2   mongod                              0x000000010646d829 _ZN5mongo12verifyFailedEPKcS1_j + 409
 3   mongod                              0x0000000105f75e1a _ZNK5mongo22GeoHaystackSearchIndex9newCursorERKNS_7BSONObjES3_i + 78
 4   mongod                              0x000000010617c21e _ZNK5mongo9QueryPlan9newCursorERKNS_7DiskLocEb + 364
 5   mongod                              0x0000000106198f09 _ZN5mongo15CursorGenerator16singlePlanCursorEv + 573
 6   mongod                              0x000000010619b8f7 _ZN5mongo15CursorGenerator8generateEv + 161
 7   mongod                              0x000000010619bc34 _ZN5mongo25NamespaceDetailsTransient9getCursorERKNS_10StringDataERKNS_7BSONObjES6_RKNS_24QueryPlanSelectionPolicyERKN5boost10shared_ptrIKNS_11ParsedQueryEEEbPNS_16QueryPlanSummaryE + 188
 8   mongod                              0x000000010607d7bd _ZN5mongo23queryWithQueryOptimizerEiRKSsRKNS_7BSONObjERNS_5CurOpES4_S4_RKN5boost10shared_ptrINS_11ParsedQueryEEES4_RKNS_12ChunkVersionERNS7_10scoped_ptrINS_25PageFaultRetryableSectionEEERNSG_INS_19NoPageFaultsAllowedEEERNS_7MessageE + 605
 9   mongod                              0x0000000106082a3d _ZN5mongo8runQueryERNS_7MessageERNS_12QueryMessageERNS_5CurOpES1_ + 7453
 10  mongod                              0x0000000105fdd94b _ZN5mongoL13receivedQueryERNS_6ClientERNS_10DbResponseERNS_7MessageE + 1131
 11  mongod                              0x0000000105fe11d4 _ZN5mongo16assembleResponseERNS_7MessageERNS_10DbResponseERKNS_11HostAndPortE + 1060
 12  mongod                              0x0000000105be52bf _ZN5mongo16MyMessageHandler7processERNS_7MessageEPNS_21AbstractMessagingPortEPNS_9LastErrorE + 261
 13  mongod                              0x00000001064b0910 _ZN5mongo17PortMessageServer17handleIncomingMsgEPv + 2384
 14  mongod                              0x00000001064aeee1 _ZN5boost3_bi5list1INS0_5valueIPN5mongo17PortMessageServer22HandleIncomingMsgParamEEEEclIPvPFSA_SA_ENS0_5list0EEET_NS0_4typeISE_EERT0_RT1_l + 79
 15  mongod                              0x00000001064aef46 _ZN5boost3_bi6bind_tIPvPFS2_S2_ENS0_5list1INS0_5valueIPN5mongo17PortMessageServer22HandleIncomingMsgParamEEEEEEclEv + 82
 16  mongod                              0x00000001064aef81 _ZN5boost6detail11thread_dataINS_3_bi6bind_tIPvPFS4_S4_ENS2_5list1INS2_5valueIPN5mongo17PortMessageServer22HandleIncomingMsgParamEEEEEEEE3runEv + 37
 17  mongod                              0x0000000106541439 thread_proxy + 169
 18  libsystem_c.dylib                   0x00007fff8514f8bf _pthread_start + 335
 19  libsystem_c.dylib                   0x00007fff85152b75 thread_start + 13
Wed Feb 20 16:42:36.178 [conn2] assertion 0 assertion src/mongo/db/geo/haystack.cpp:178 ns:test.c query:{ query: { pos: 4.0 }, $explain: true }
Wed Feb 20 16:42:36.178 [conn2] problem detected during query over test.c : { $err: "assertion src/mongo/db/geo/haystack.cpp:178" }