When building a cable, a heat gun was used to melt & shrink a one-piece connector connector. The heat did its job but it also heated the insulation of the wires a little further away from the joint. Two adjacent wires got so hot their insulation melted and the conductors inside shorted. From the outside, it looked good (of not a little shiny from the partially melted plastic), but inside there was a short.
After making a cable, we should test them for continuity and shorts.