I found out that a problem I was having was fixed in the release version of an object I'd been editing an early version of. Oops. Time to do it over, with any luck better this time.
So... I made a working copy of it, and set to editing. I'd been told it helped to place the object on the ground/floor to work on it, so I did, and it did help.
After some work, it seemed to be in reasonable shape—putting it on and comparison with the previous version seemed to check out—so, it was time to make copies and paint them with alternative textures...but I couldn't! Looking in inventory, it was marked "no copy" and "no modify." "No modify"?! I'd just been editing it!
I resigned myself to starting over—practice makes perfect, right? This should get easier each time I do it—and suddenly the light came on. I created a working copy once again: it had the same permissions as the original. I set it on the floor: poof! It changed permission.
Now I know what sandboxes are for. :)
UPDATE: No, I don't. It happened to me again, and a friend told me that something similar happened to her. I have also looked closer--the inventory says the object is not modifiable, but if you edit it, the object itself says it is modifiable, and that appears to win in a fight between inventory and object. OTOH, inventory seems to be the one with the info on copyability, and its version of "modify" permission is all that's looked at to decide whether to let you rename. I am now wondering whether there was a change to fix the disappearing object problem that had this as a bad side effect. That's just as much as guess, though, so I'll search JIRA and then decide whether to submit a bug report.