That was one of Frank Avingdale's [sp.] stories.
He would take a stack of blank deposit slips, bring them to his home where he MICR encoded the bottom with his account number, and return them to the "island".
Customers would fill in the deposit slips and not really aware/concerned of the MICR encoding. Since they were already encoded, the tellers would just process them. The deposits (after they ran through proof) would end up in his account.
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Integrity. With it, nothing else matters. Without it, nothing else matters.