Edison Movie reference - 1940 

I just heard a bug reference in the movie - Edison, The Man (1940) https://en.wikipedia.org/wiki/Edison,_The_Man - towards the end of the movie when Edison determines that his two dynamos have a problem because the speed governors were not synced until he mechanically linked them. 

His first erroneous attempt caused the dynamos to act as a motor/generator combo until the governors were mechanically linked. He wanted the dynamos to work as parallel generators. In the movie, Spencer Tracy distinctly used the term "bugs" for this problem when he first attempted to light up New York City.

Granted, this is a movie. But the term "bug" was obviously used during Edison's invention period for mechanical design issues that needed to be resolved and thus included in the movie. Re-quoting the previous Edison quote - ["Bugs" — as such little faults and difficulties are called]. And the movie itself is from 1940, which is earlier than when it was first used for software. It seems likely that "bugs" was an engineering term before it made it into software.