As usual, the software-development process became a much greater consumer of time than anticipated, and unfortunately the stress was a cause of a lot of broken marriages. Interestingly, programmers were given a job with almost no description, so they made it up as they went. One of the developments was a multitasking system that assigned priorities to tasks, so that when the computer got overloaded and couldn't keep up with all of them, it purposely neglected lower-priority ones in order to keep more critical ones going without delays, something that the astronauts' lives depended on particularly when landing on the moon.

Prior to execution, but subsequent to being fetched from memory, an instruction could be modified by adding a value computed at run time. It was a sanitized version of self-modifying code. IIRC the main intent was simply to implement indexed addressing. The addition was expected to modify an Address Field located in the bottom of the instruction word.

That's clever in itself, but a fascinating quirk yielded capabilities far beyond what had been originally planned. Someone realized that an addition could also or alternatively modify the Opcode Field in the top of the instruction word. Moreover, such an addition would produce an n+1 bit result -- a carry. This anomaly was recognized as an opportunity. The carry was decoded along with the original opcode bits, and the opcode space was thus doubled. A whole new set of instructions could then be defined -- instructions which were only available via this curious back door technique!