Read 30 lines of code and find the race condition, N+1 query, SQL injection, or off-by-one error.
The code compiles and usually passes a happy-path test; the defect is the kind that reaches production. Of four plausible readings, one is the actual bug, and the explanation covers why the others are tempting. Cards are scheduled with spaced repetition: missed cards return sooner, and known cards space out.