There is an edge case, which is easier to reproduce on Safari when user will open a few tabs with running webcontainer (tutorial pages, homepage if it's scrolled to embedded editor or playground).

When an OOM error occurs in Safari, the tab is reloaded automatically in that case event beforeunload was not catched and count of running webcontainers wasn't decreased.

In the result the alert message:
You currently have more than 3 tabs open running the Angular Tutorials or Playground, this may result in an <code>Out of memory</code> error.
could be displayed even when user has less opened tabs with Embedded Editor than recommended.

The idea of solution:
I'm thinking about reset counter when will close the warning message.