"Working overtime is like throwing extra distance into the next race. One of two things is going to happen. Either the person is going to push themselves like the normally do but become exhausted by the end (in this case, they'll complete the extra features ok but will suddenly slow down on Monday when they're supposed to be working on the "main" development), or they'll adjust their pace to suit the extra hours. While you used to budget 8 hours for a certain kind of task, it will now take them 9 hours to do it because they're going slower. "
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Please review the linked page for context.
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