ENIAC, an acronym for Electronic Numerical Integrator and Computer, was the first high-speed fully electronic general purpose digital computing machine.

Built for the United States Army's Ballistic Research Laboratory, with its intial task being the calculation of artillery firing tables.  But it was unique because rather than being capable of being used for only one single purpose, it could be "programmed", and reprogrammed, to solve a full range of computing problems.

ENIAC filled an entire room, weighed in just short of 30 tons, and at a time before 'transistors' relied on vacuum tubes - thousands of them.

One of the problems with vacuum tubes is that they heat up. Warm vacuum tubes, emitting light, attracted moths.  Flying - and dying - moths had a tendency to interfere with the operation of the tubes.

To ensure proper operation, ENIAC was regularly 'debugged'.

The vaccum tubes and moths are long gone. 
The term 'debugging' is still in use.