Comet tail artifact is a form of reverberation artifact. In comet tail artifact the two reflective surfaces are closely spaced together (such as the bevel of a metallic needle). The reflective surfaces are so close that it is difficult to distinguish between each reflected echo.
Comet tail artifact is different from ring down artifact (described next) because the comet tail artifact dissipates with depth and has a triangular and tapered shape. See the image below of a comet tail artifact arising from a needle tip.