I made a quick but not completely disposable scripted object. Probably 30-45 minutes of work. I saved it in the embedded library of a scratch file, rather than in a live server folder where someone might notice it before it's done. (The odds of this are crazy low, plus it's no big deal, but that's what I did.) The scratch file is Archicad 20, because we haven't officially migrated, but my projects are in 21.
Then the standard procedure is to save the object out in Library Manager, and delete it from the embedded library so the scratch file doesn't have a duplicate.
Then, reload libraries in the real project so the new object can be seen.
Upon reloading, I had several duplicates. Inspection revealed I had exported the whole embedded library from the scratch file. (The two files have some deprecated labels in common.) I had clicked the save button without highlighting the single object.
That's the kind of thing I like to fix right away, so I went to the Finder and found the stupid 'Embedded Library' folder inside '22 Plumbing' and deleted it. Because this folder was on a server volume, the deletion was immediate - such things don't go to the trash. (I still miss this from OS9.)
Then I went back to the scratch file to export the object properly. But since my procedure is to delete the object right after saving it, it was already gone.
It's gone from the embedded library, and the server copy was deleted along with the stupid folder. Maybe it still exists in the actual project, since I hadn't reloaded there yet. Alas, no, because while the object still appears in the settings dialog, it doesn't work at all because the file on disk is missing. I also tried crashing the scratch file's Archicad instance in hopes the autosave data might remember the object. Alas, no, again.
So that is how I managed to completely delete a new object. The only hope would be if Time Machine or Backblaze had run during the few minutes between saving the embedded library folder and deleting it. The odds of that are also low, and it didn't happen.
It's frustrating to lose work, but it wasn't a lot. Between automatic backups and autosave, it's difficult (knock wood) to lose a lot of work in Archicad. Autosave for objects would be nice, but it wouldn't have helped in this case. While I've been at this a long time, I still find new mistakes to learn from.