There is a litany of potential Y2K effects. Electronic data interchange fails. ATM machines won't operate. Centenarians are invited to register for kindergarten. Machines stop because embedded chips show cycle times have been exceeded.
Credit cards expire.
Receivables are a 100 years overdue. Three types of failure. The first and most likely problem is that your software will see 00 as 1900 and not 2000. The third is that your computer stops. Y2K is the ultimate business challenge.