I had a 8 digit PIN, but only the first 4 mattered to the ATM. The screen would blink after the first 4 numbers, too, to help you notice you already had it.

Then two weird things:

1. The bank got bought by someone else, and the new bank demanded 6 digits. I had stopped using anything past the first 4, but they were still the first six from the original form I filled out years earlier.

2. They sent me my full PIN in the mail. At first I was surprised by this, since one-way-hash and all that jazz, but on reflection one-way hashing a six-digit PIN is pretty silly.