thought i would do a test in mame to see if i could get it to happen but no occurrences when i tried (USA 910206) i did see something i hadnt seen before; when the elephant at the back has his trunk in the middle position you get a black pixel appear just below his ear (left red arrow).. also you get two grey pixels appear on the background at the base of the pillar to the right (right red arrow) tried 'World 910522' and got the same thing, so assume all is normal