Quoting wimvdb (one of the most thorough and knowledgable testers/users - he's in the splashscreen credits) from November 2012:

Quote - Apparently it is not easy. I filed my first reports about this in 2006 and a couple of others after that. Each time we thought we had them all - all my test cases had it fixed. Then - after a while - you discover another case. New cases were discovered after the introduction of HW Shading, python enhancements, firefly optimizations and a couple of other things. All variations of NO_MAP were corrected as far as we could think of (no_map, NO_MAP, "NO_MAP", "no_map", NOMAP, "NOMAP", "", "  ", '', file "", file NO_MAP, file "NO_MAP" and more which I can't remember.
In the current release I have not seen the problem at all, so I can understand that it is difficult to track down. I use a LOT of old content all the time, so it seems most of the occurrences are handled correctly. I have the suspicion that there is another factor involved. Something like resaving the object to the library or something else which is not very obvious.

Quoting infinity10, same day:

Quote - I also received advice from Smith Micro Tech Support.  I am mentioning it here for benefit of other customers / users who encounter the same problem.
Problem arises if 2 conditions are met:
Hardware shading is turned on, AND content has image node but no image has been added.
So it is a content-driven issue, but NOT a fatal error if hardware shading is turned OFF.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)