I'm imagining it was a case of an SQL-based password check where "TRUE OR" got added to the WHERE clause, and the code takes the first result instead of expecting only 0 or 1 row.
Are there other easy ways to do this?
2) Mentioned elsewhere in this thread, a variable with global scope within an application server. This is very possible in node.js, which uses a long-running single thread - if you have a function like handleRequest(), you might inadvertently write to a global variable outside it, and that variable will persist across requests from different users. I've seen this exact bug in a PR - luckily we caught it before production, but if it had slipped through code review and integration tests and actually shipped, the result would have been exactly like the one in the tweet.