Sometimes the key leads oxidize and swell, even on Steinways. Mind you, the originals hadn’t swelled as much as those added after new hammers were put on. Still, they had all swelled a bit – and some enough to rub on neighbouring keys:

lead protruding 2m

Some keys have hideous amounts of lead in, so filing them flush with key sides is messy (nasty work, if you can get it!):