She was in southern France celebrating her boss’ birthday.
Mail Online, October 23, 2017
This shows abysmal ignorance, even by Mail Online’s uniquely low standards. (It appears in both the standfirst and the story, so it is not a typing error.) Presumably the person responsible thought that because ‘boss’ ends in ‘s’ it is a plural, so should be treated like, for example, ‘the drivers’ cars were all towed away’. Of course ‘boss’ is singular, and is treated like any other singular noun, adding an apostrophe plus s, so it should be ‘her boss’s birthday’.
An exception is the expression ‘for goodness’ sake’, which means ‘for the sake of goodness’ so needs an apostrophe. Technically it should be ‘for goodness’s sake’, but it has long been established in its familiar form.