In 1969, László Bélády and two IBM colleagues published a paging-machine anomaly showing FIFO could make four memory frames suffer ten page faults after three frames suffered nine, leaving generations of operating-systems students staring at the moment more memory became the wrong answer

László Bélády’s anomaly begins with a result that still looks like a mistake when students first calculate it: the page-reference string 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5 produces nine page faults with three FIFO memory frames, then ten page faults with four. One extra slot makes the machine miss