`  ==least surprise??==  So am I the only one who thinks this is an entirely bogus ``principle``? What surprises one person is often perfectly expected to another one. In the second example, I'd be quite surprised if I issued a shutdown and the system stayed up (because someone else issued a shutdown concurrently to me and then later canceled it). In this situation there is no behaviour that will not surprise one of the two users.   I'd go so far as to assert that every single example of a true violation of this ``principle`` (i.e. a situation in which it can be unambiguously established which of two different behaviours is more or less surprising) is simply a bug in the underlying program.  Can anybody name any such situation where we're NOT just talking about a bug?   `