Read 30 lines and find the race condition, N+1 query, SQL injection, or off-by-one. Four choices, one real bug.
The code compiles and usually passes a happy-path test, the defect is the kind that reaches production. Four plausible readings are offered and only one is the actual bug; the explanation covers why the others are tempting. Cards are drawn with spaced repetition, so the ones you miss come back sooner and the ones you know back off. Progress is kept in your browser.