1) Create a screen with one element. A graph, for example.
2) Create a slideshow using that screen.
3) In user profile set page to refresh 30 seconds.
4) Open Monitoring > Slideshows and select it.
5) Click mouse outside browser, so that tab or browser window is not in focus.
6) Wait for 2 refreshes or about a minute.
7) Notice JS error TypeError: timer is undefined starting to show up.

If screen has more elements, error is generated for each element. Bringing focus back to tab or window, continues to generate same error.