[simcop2387]

It doesn't have to be going the exact same speed, a multiple would do the trick too. It can also be dangerous if it's close to the same rate since it can then look like it's going slower even if not stopped.

[TheSpiceIsLife]

Or the rotating / reciprocating object may appear to be rotating / reciprocating slowly in the opposite direction.

Even after 20 years in the metal fabrication industry that scenario still spooks me.