As I remember, this was with Comcast's just released 4K box. Obviously I had no prior experience with this unit. Evidently, neither did the trainer.

Comcast trainer and a trainee were on site and struggling without results. We were close by and the customer called us. Shortly we had the two installers and my partner struggling without results. I, on the other hand was off in a corner, apparently amusing myself. I was actually reading the installation manual packed with the unit. The instructions clearly stated "install fiber washer here". I rummaged through the packing materials and there was the washer -- still in its pack. After installing the washer "here", everything worked.

This unit had a plug-in IR unit on a mini-phone plug. The shaft on the plug was too long or the jack was too short. The washer spaced the plug out of the unit enough to work around the issue.