I think it’s a fairly rare case, but still… 
Theme components often have this button to install the theme component on a remote Discourse:
Here, I entered discourse.canapin.com instead of discourse.canapin.dev. The first domain leads to a non-working IP, a remnant of past experiments.
Then, there’s no error message. The “Install” button stays greyed out, and nothing happens:
After a while, https://meta.discourse.org/theme-install-checker.json obviously returns a 502 error, but it’s only visible in the network tab of the browser’s dev tools.
I’m not sure something should be done about this, but since I keep mixing up my domains, it led me to this situation a couple of times and I didn’t get what was wrong (because there was no error displayed).
Maybe a message should be displayed, whatever the error number is.
Maybe a loading icon should be added next to “Install” in the button if the operation takes more time than usual.
edit: I saw that there’s a cool loading icon on the fast edit button. Something like this would fit:
