Re: Has anyone ever seen a non-disparagement clause in their contract ?
In the UK I think they (and NDAs) are common in "redundancy settlements". In other words, where the employer wants staff to leave without fuss and offers to pay them more than the legally required amount of compensation. Sometimes it is for a particular individual (often senior, or well known in their market). Sometimes it is tied to other voluntary agreements such as agreeing not to work for a competitor for a while.
I have never seen one in an employment contract, though.
Biting the hand that feeds IT
