I was replacing a failed breaker last week and noticed a gray film on several of the wires in my entrance panel. This film seemed to be on the feeds from the meter so I pulled the meter. One of the load side aluminum lugs had melted and the aluminum wire connecting the main breaker to the meter base had been "welded" in place. Not a pretty sight since the only protection I'm aware of is a 10 amp fuse in the primary (13 KV) side of the power company's transformer. Burning up an underground line from the transformer to my meter base would not have been fun.
The moral here is to look carefully at conditions in the panel when working there. I had no symptoms, no flickering lights and no variation in voltage with load but still had an unstable situation.
I now have a new meter base and copper wire.