http://buildlogs.mongodb.org/build/51824031d2a60f31d2000107/test/5182e052d2a60f407500080c/
Thu May 02 18:42:27.713 [conn13] KERNELBASE.dll DebugBreak+0x2 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\util\debug_util.cpp(46) mongo::mongo_breakpoint+0x22 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\util\debug_util.h(65) mongo::breakpoint+0x25 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\util\assert_util.cpp(121) mongo::verifyFailed+0x2b0 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\bson\bson-inl.h(183) mongo::BSONElement::embeddedObject+0x6a m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\commands\mr.cpp(845) mongo::mr::State::finalReduce+0x622 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\commands\mr.cpp(1266) mongo::mr::MapReduceCommand::run+0x151b m31100| Thu May 02 18:42:27.713 [conn124] warning: we think data is in ram but system says no m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\dbcommands.cpp(2015) mongo::_execCommand+0xb0 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\dbcommands.cpp(2126) mongo::Command::execCommand+0xb19 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\dbcommands.cpp(2218) mongo::_runCommands+0x3f6 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\ops\query.cpp(50) mongo::runCommands+0x7b m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\ops\query.cpp(955) mongo::runQuery+0x48c m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\instance.cpp(235) mongo::receivedQuery+0x2f0 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\instance.cpp(374) mongo::assembleResponse+0x353 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\db\db.cpp(194) mongo::MyMessageHandler::process+0x133 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\mongo\util\net\message_server_port.cpp(206) mongo::PortMessageServer::handleIncomingMsg+0x676 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\third_party\boost\boost\bind\bind.hpp(244) boost::_bi::list1<boost::_bi::value<mongo::PortMessageServer::HandleIncomingMsgParam * __ptr64> >::operator()<void * __ptr64,void * __ptr64 (__cdecl*)(void * __ptr64),boost::_bi::list0>+0x56 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\third_party\boost\boost\bind\bind_template.hpp(21) boost::_bi::bind_t<void * __ptr64,void * __ptr64 (__cdecl*)(void * __ptr64),boost::_bi::list1<boost::_bi::value<mongo::PortMessageServer::HandleIncomingMsgParam * __ptr64> > >::operator()+0x61 m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\third_party\boost\boost\thread\detail\thread.hpp(63) boost::detail::thread_data<boost::_bi::bind_t<void * __ptr64,void * __ptr64 (__cdecl*)(void * __ptr64),boost::_bi::list1<boost::_bi::value<mongo::PortMessageServer::HandleIncomingMsgParam * __ptr64> > > >::run+0x2f m31100| Thu May 02 18:42:27.713 [conn13] mongod.exe ...\src\third_party\boost\libs\thread\src\win32\thread.cpp(185) boost::`anonymous namespace'::thread_start_function+0x4c m31100| Thu May 02 18:42:27.713 [conn13] writing minidump diagnostic file mongo.dmp
according to mathias, probably this link: https://github.com/mongodb/mongo/blob/master/src/mongo/db/commands/mr.cpp#L845