Why do we make mistakes in repetitive tasks?

Why with repetitive tasks do we make mistakes?

For example, put the plastic clothes pegs in the black bucket and the wooden clothes pegs in the red bucket. That is plastic - black : wooden - red. For the first 27 you are okay and then you put a plastic clothes peg in the red bucket !

Just a simple example, but in some tasks, such as pharmacy dispensing, if you give old Mrs Brown too high a dose she might kick the bucket (either black or red!)! Perhaps that is not quite the same example because patients have different prescriptions, but offhand I cannot think of another suitable example.

Is it brain fatigue or does your brain go onto "auto pilot"? I wonder if the mistake rate would drop if it were plastic clothes pegs - plastic bucket : wooden clothes pegs - wooden box.