The article touches on this:

All told, 69 of the 300 items came up higher at the register: a 23% error rate that exceeded the state’s limit by more than tenfold.

This implies that an error rate of perhaps 2% would be legal. I haven't checked, but I guess Europe has something similar even though I'm quite certain that retailers have to sell things at the posted price if there's a mistake.

Part of the problem seems to be that the maximum fine (at least in the state in the article) is "too low", so retailers don't have an incentive to keep price tags correct since they profit from the error and even if they're fined it's still better (economically) for them to charge more than the price tag. I wonder how much lobbying has happened to keep fines low ...