Trying to add a post to a set when it's already there now pops up a message saying Post #<id> added to set 'undefined'. Before, it correctly showed a red alert instead, and said something like '#<id> is already in the set'.

What part(s) of the site page(s) are affected?
https://e621.net/post/index/

Can you reproduce the bug every time?
Yep.

What steps did you take to replicate this bug?
Post index -> Mode: Add to set -> click on the thumbnails

+1 I just noticed the same thing.

i have also noticed this bug.

Thanks for the report. It turns out that fixing the error handling on the JavaScript side uncovered an even more annoying bug. The generic error reporting function used has been returning the wrong HTTP status code for years. I've put in a fix for this, and it should be deployed soon. In addition to sets, it should also resolve a number of other areas returning incorrect success notices as well.

Edit: Deployed.