The title sounds very wired, but it is true :-) I have three different external monitors (in different offices) and two of them run very well. Only one (a Dell U2711) causes a strange behaviour on my MacBook: it logs off my user non-periodically! While I'm working, suddenly I see the login screen and after a login, all my programs are closed. When I open the applications again, they say "was not closed normally", for instance Google Chrome asks me whether it should re-open all tabs. This happens only, if I use this monitor. It is connected via DisplayPort.