Description: These heiroglyphs are found on the ceiling beams of a temple in ancient Egyptian Abydos and can still be seen in-situ today.
Mainstream Theory: The glyphs are a result of both erosion of the stone surface (evident elsewhere in the temple) and the process of filling in and re-carving the stone to replace some of the original hieroglyphics. The technical term for such a surface that has been written on more than once is a palimpsest. The usurping and modifying of inscriptions was common in ancient Egypt throughout its history. The Abydos glyph was modified at least once in antiquity, and perhaps twice. Some of the filling has fallen out in places where the older and the newer inscriptions overlap, and the result is unique and odd-looking.
Alternative Theory: If these images are merely "random, fallen plaster" creating this illusion in our minds, this would be a statistical impossibility as we're not just seeing one "illusion" here, but at least four, four very sophisticated representations of four different types of the same "impossible" mechanical technology all grouped together on one panel.