A load of old washers
I used to re use the old ally sump plug washers until I got badly caught out. I'm going back to 1979 so, yes, it has scarred me for life!
What happens is that they crush a little each time you use them, so after repeated re using they become significantly thinner.
A lot of sump plugs have an undercut at the bottom of the thread, where it meets the flange or flat face face of the plug hexagon. But not all have the undercut. Some just have the thread stopping a couple of mm short of the bottom, but with no undercut.
With this type, if you allow the washer to get too thin, the thread will run out and the plain portion will enter the tapped hole and split the sump.
Be very afraid.