It happens to me as well (Windows, Chrome) but inconsistently (sometimes, its on time).  Looking at the JS, it might be since it schedules to check for updates every 100ms (`timerId = setInterval(update, 100);`) instead of scheduling a callback for the time of the next bar.<p>Edit: Also, the JS `setInterval` callback can be delayed.<p>Edit: After debugging their code some more, the above isn&#x27;t why (though is still inefficient).  The audio player (SoundManager) they are using only updates the audio playback time at a rough granularity. If I log the time values, I get the following (all in milliseconds):<p><pre><code>    0
    182.404
    681.59
    1180.778
    1680.963
    2179.152
    ...
</code></pre>
The song is 120 BPM which means that the first bar changes is at 2000ms causing the first bar change to be 179ms late.