As an operator in a chemical plant, in the middle of winter in Indiana, I have seen centrifugal pumps that have been dead headed and running for as little as 1/2 hour cavitate due to the fluid boiling from circulating and not moving. These pumps are attached to the motor via a coupling so heat transfer from the motor is not likely. If ran for a very long time, the pumps would be so hot that you couldn't touch them and the pain would discolor.

Friction between molecules will cause the fluid to heat up.