I've never seen one of those threads. More common are the "I just had my shop put on my cog and lockring, and from skidding around on it all day with play in it, now it's stripped; it turns out my shop doesn't know dick about fixed-gear street riding" threads.

And, because I'm ****ing sick of seeing it, it is rotafixa, it is an italian word. It is not some corruption of "roto-rooter". You can rotafixa your cog on. THERE.