Measure Twice Cut Once

This one is pretty tough to learn and tends to be regularly forgotten, to be learned again. There is always a point where the will to finish overcomes the standard precautions.

It is wise to double check your measurements-markings before performing a non cancelable operation like cutting or drilling. The amount of verification should be proportional to the sensitivity of the operation, scarcity or price of a material, previously invested time in the the part you are machining…

Example : you will check 36 times your marking if drilling in a piece of marble that you machined for 22h prior to that and for which you have no spare material.