Shaft bow
I recall having to motor 2 engines at a time (only 2 hands!) during turnarounds, to cool the engines below 100C EGT, to prevent shaft bow on subsequent start up. Was a requirement to prolong engine life if I recall.
Also remember pulling multiple CBs in the cruise coming eastbound during the night, showing new FEs the tandem bleed system in action, in case they were ever needed to do a test in anger. Had no clue at time of how risky this regular practice was, risking a major engine malfunction if the bleeds went wrong!!