A small one, but one that led to quite some confusion.
When a broken theme component is detected, a red bar appears stating “Your site may not work because theme / component %{theme} has errors. Disable it at %{path}.”
However, since (in this case) Discourse also detected that this theme component was not on its latest commit, the correct action here was “update” instead of “disable”.
In my experience that is the case for the majority of situations where this message appears. The quick fix would be to update the text to “Update or disable”, a more extensive fix would be to vary the message depending on whether there is an update available for the theme (update) component or not (disable).
It is now the default, but we didn’t change existing themes (because historically people might have made local modifications to them, and we wouldn’t want to automatically overwrite those changes)