I suspect it is an Echinometra - harmless (a "rock boring urchin" and common hitch hiker...don't know if you got it that way).
Several species, Echinometra lacunter is a frequent one.
Species of concern would be in the genus Toxopneustes, or Asthenosoma (a very pretty urchin and it wouldn't surprise me to see it in the hobby some day).
The long spined urchin Diadema is a known issue but no where near as problematic as these would be. When I hear someone describe a species that has short spines (as in the original thread) and they are told it is highly venemous...I panic a bit.
In this case it was probably (though I don't know for sure) something not venemous at all and just a common urchin, with apparently a misconception on the part of the LFS (no!!!). It is always wise to be careful handling urchins, for their sake and ours...but many are quite harmless.