db.users.find({name: 'Manuel'});

{
  _id: 50b17cdc093fbebe6a000001,
  name: 'Manuel',
  pass: '',
  clients: [],
  channels:
   [
     { time: 1353933188626, socket_id: 1, channel_id: 1 },
     { time: 1353933189784, socket_id: 2, channel_id: 2 },
     { time: 1353933190679, socket_id: 3, channel_id: 3 },
     { time: 1353933191568, socket_id: 4, channel_id: 4 },
     { time: 1353933192462, socket_id: 5, channel_id: 5 },
     { time: 1353933194112, socket_id: 6, channel_id: 6 },
     { time: 1353933195049, socket_id: 7, channel_id: 7 },
     { time: 1353933196044, socket_id: 8, channel_id: 8 }
   ]
}

// works:
db.users.update({"name": "Manuel"}, {$pull: {"channels": {"time": 1353933189784, "socket_id": 2, "channel_id": 2}}});

// does not:
db.users.update({"name": "Manuel"}, {$pull: {"channels": {"time": 1353933189784}}});

// neither does:
db.users.update({"name": "Manuel"}, {$pull: {"channels": {$elemMatch: {"time": 1353933189784}}}});