Eventually he realized that Kaye was using self-modifying code to process elements of an array, and had coded the loop in such a way as to take advantage of an overflow. Adding 1 to the address field of an instruction that referred to address x normally just changed the address to x+1. But when x was already the highest possible address, not only did the address wrap around to 0, but a 1 was carried into the bits from which the opcode would be read—in this case changing the opcode to "jump to" so that the full instruction became "jump to address 0".
Eventually he realized that Kaye was using self-modifying code to process elements of an array, and had coded the loop in such a way as to take advantage of an overflow. Adding 1 to the address field of an instruction that referred to address x normally just changed the address to x+1. But when x was already the highest possible address, not only did the address wrap around to 0, but a 1 was carried into the bits from which the opcode would be read—in this case changing the opcode to "jump to" so that the full instruction became "jump to address 0".
— https://en.wikipedia.org/wiki/The_Story_of_Mel