If communicated and part of the deal, great. I personally think that an employment benefits both parties. And with the mentioned curtisyz that works well.

For example, I leave early for appointments, in the last weeks we had some troubles, so dinner for the hole family was on the employer, a while week of takeouts.

So, if my employer tells me that my vacation colides with a project, I am certain that he checked every possibility, and we try to find solutions, like interruptijg the vacation for a day and taking part in meetings from my hotel room.

And if I can not trust my employer enough, then I move on. I am in the lucky position that the stuff I do, most people can't.