[Someone1234]

I'm sure all of them thought about it. But were then told the terminals used for scan & shop or the self-scan kiosks needs a bit of slop in them to make them appear more reliable, meaning to work "offline" for a short span of time. It is a trade-off between serving real customers (meaning the majority) well but with the downside of benefiting fraudsters.

All systems have trade-offs like these. It reminds me of the phase: "Anyone can build a bridge, but it takes an engineer to build a bridge that barely stands." That applies here. Any student can build a system with locking database records, but then when thousands of people's cards don't work for minute-long lockout periods, you aren't the one doing the CS calls or getting yelled at.