run in the shell:
> while( 1 ) db.getLastError:

go to web UI hold down reload. you will get something like the following. i assume a concurrency problem maybe the query object has been destrictured when this is accessing

mongod.exe!mongo::breakpoint() Line 79 C++
mongod.exe!mongo::msgasserted(int msgid, const char * msg) Line 110 C++
mongod.exe!mongo::BSONObj::toString(mongo::StringBuilder & s, bool isArray, bool full) Line 237 + 0x2a bytes C++
mongod.exe!mongo::BSONObj::toString(bool isArray, bool full) Line 219 C++
mongod.exe!mongo::operator<<(std::basic_ostream<char,std::char_traits<char> > & s, const mongo::BSONObj & o) Line 580 + 0x13 bytes C++
mongod.exe!mongoutils::html::tablecell<mongo::BSONObj>(std::basic_stringstream<char,std::char_traits<char>,std::allocator<char> > & ss, const mongo::BSONObj & t) Line 64 + 0x41 bytes C++
> mongod.exe!mongo::ClientListPlugin::run(std::basic_stringstream<char,std::char_traits<char>,std::allocator<char> > & ss) Line 406 + 0x35 bytes C++
mongod.exe!mongo::WebStatusPlugin::runAll(std::basic_stringstream<char,std::char_traits<char>,std::allocator<char> > & ss) Line 303 + 0x13 bytes C++
mongod.exe!mongo::DbWebServer::doRequest(const char * rq, std::basic_string<char,std::char_traits<char>,std::allocator<char> > url, std::basic_string<char,std::char_traits<char>,std::allocator<char> > & responseMsg, int & responseCode, std::vector<std::basic_string<char,std::char_traits<char>,std::allocator<char> >,std::allocator<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > > & headers, const mongo::SockAddr & from) Line 248 + 0xc bytes C++
mongod.exe!mongo::MiniWebServer::accepted(int s, const mongo::SockAddr & from) Line 138 C++
mongod.exe!mongo::Listener::initAndListen() Line 214 + 0x1d bytes C++
mongod.exe!mongo::webServerThread() Line 540 C++