Can Windows 2008 R2 have different service packs on domain controllers

Just wanted to throw out a question to get opinions...  this is regarding peer domain controllers on the same domain, all with Windows 2008 R2.
Does it seem ok that one should have Service Pack 1 installed on it, and the other 2 do not have the service pack installed? I guess there could be a lot of reasons for this. Mainly, there is a window to do the installation on one of them, but not sure when we can do the others. Would this be harmful, or should they be kept in sync with the same patches and service packs?  Comments appreciated from those who feel they understand this well. Thanks.