MQTT topic received by Espruino sometimes malformed

It was just this:

var mqtt = require('mqtt')
var client  = mqtt.connect('mqtt://localhost')
 
function go() {
  client.publish('random', ""+Math.round(Math.random()*10))
}
 
setInterval(function() {
go();go();go();go();
}, 5000);

Basically sending a bunch of MQTT info at once so it is bundled up and sent in one packet.