I have not seen this before but it seems rather wrong, the screw and the washer were not hot, but why would one do this on the main breaker.
There were other electircal issues going on at this place so a call out to an electician, should this also be flagged, just seems so wronb to have something like that between the breakers
This appears to be a breaker designed to be held in place since it is a 100A breaker, It must be connect securly to the panel enclosure. This is done to keep the breaker from popping out due the increased size of the wire plus the NEC calls for it to be screwed in is back feeding a breaker used as a main.
Sorry I am being brief and lax on the explanation…I will do better when I am back at the office as I am tapping this in from a job on my pocket pc.