How does this happen?

Not the person you are asking. Bot operators have an incentive to make crawlers look as much like a human as possible so they do not get blocked. Some of them fail miserably and some nearly succeed. That makes it trivial to accidentally block a real person. I am personally fine with that given I do not pay for this site and have no SLA or contract with it.

Every filter process has false positives and false negatives, especially when crawlers are trying to fake their status.

Last week if you are using a VPN + a browser that limits fingerprinting, you were likely to see error messages accessing HN.