IMO just don't use that stab.
They likely just moved the breaker which needed replaced and it just started working on the next spot it was installed at.At my son's school there is a 16 space panel that has 11 breakers and 7 blanks. I opened it up to look because gymnasium lights were flickering.
Bus was burned and pitted at that breaker, so I looked at unused spaces and they were all the same. Good grief, somebody's been doing the breaker shuffle for a long time....:huh:
It's well past time for a panel change
In the bad old IT days of removable hard disk packs a head crash would happen occasionally, damaging both the head and the disk.They likely just moved the breaker which needed replaced and it just started working on the next spot it was installed at.
Sometimes if there is enough heat, other areas of same bus are effected.
In a dwelling you often just see one pole that is effected, in other then dwellings where there may be significant continuous loads I have seen multiple poles that are compromised, even though just one of them may be the culprit that started everything.