Updating a UserCSS.
The local version was newer than the online one, so it didn't update. You click the force update button and nothing happens. Took me a good five minutes to find out what was wrong and that there indeed was an error message, but hidden as a tooltip on the button. The tooltip won't show if your pointer rests at the button you just clicked.

Why not throw an alert?

In addition, the error message is not translated.

Stylus 1.4.13
FF 61.0.1