If you err, you make a mistake. It criticises the main contractor for seriously erring in its original estimates If you make a threat be sure to carry it out if he errs again
wander from a direct course or at random; "The child strayed from the path and her parents lost sight of her"; "don't drift from the set course"
If you err on the side of caution, for example, you decide to act in a cautious way, rather than take risks. They may be wise to err on the side of caution He probably erred on the conservative rather than the generous side