I determined that it was failure of the MOS Technology 6502's which were supposedly capable of 2 MHz operation but couldn't if they got a little static electricity jolt. I remember one time at the Homebrew Computer Club having to swap the processor with one that I had in my pocket. By the time we went into production we had processors that didn't have this problem. That may sound like a lot, but the hardware was the easy part for me. The software that I wrote was very tight because ROMs were small and expensive back then.
Software is much more flexible than hardware, exhibiting many ways be which to shorten code.
I'd use the same techniques for my machine language software that I used with hardware. That's why I included some hardware examples that you wouldn't necessarily have understood. This made me extremely intimate with the programs and with the processor. I could carry around programs in my head and come up with ideas for improvements. Of course I'd have program problems to debug.