Electrician here, that's heat scoring. Happens when the terminal is loose, or the draw is nearing the upper limit of the installed wire for an extended period of time (looks like #8 TW, which is not ideal to run at 50a for more than 3 hours continuously, especially for how old it is). It could also be that the breaker is old enough to not be tripping reliably. Either way, there's enough resistance with that scoring that I'll just keep getting worse and continually building up heat until it melts the breaker and/or burns off. An electrician needs to swap that breaker and re-pull the wire (with #6s), that's not a utility issue.
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