The problems aren't trivial, and sometimes the solution isn't trivial, but that doesn't mean there aren't strategies that automatically mitigate some of the issues. Using database transactions, verifying things worked correctly, rolling back transactions with unexpected results, not making assumptions like "two users won't change passwords in the same millisecond", and so on can protect your product. PHP and MySQL are largely to blame as so many web developers started out building sites on a database that didn't support transactions if you used the default engine.

Basic software quality assurance is missing from the majority of web development businesses.