@sjw
1) people still don't seem to be able to comprehend just how much manpower the pharaohs had at their disposal to throw at making the pyramids. Enough that it doesn't matter that your cutting rate is only 4mm/h.

2) the people doing the labor weren't educated. They wouldn't have known/realized they weren't cutting the piece correctly. They would have relied on some supervisor to inspect things from time to time. It's not unfeasible that from time to time you get the inspection delayed by enough time to get major errors.
The actual skilled workers were tasked with doing the carving and sculpting, not the ones just cutting a stone block.

3) the reality is we don't know why that stone piece was abandoned. We assume it's because of what we view to be an error in the cut, but that's just our speculation.

4) the cutting method I heard Egyptians could have used had nothing to do with copper saws, rather just using the friction of a string and sand, that you'd continuously pour in the cut to increase abrasion.