Encounters that are unwinnable until you come back later with a higher level party and better equipment are an interesting challenge, but an encounter that is unwinnable no matter what? That's very hard to do without ending up as a source for complaints.
In my opinion a good unwinnable encounter was the Lady of Pain from Planescape Torment. You are given plenty of warning that she can kill anything she deems a threat (she even killed a god once) and you won't encounter her at all unless you go out of your way to provoke her. And even then she will only kill you if you provoke her twice, with you having no reason to do so other than wanting to see what fighting her is like. I should also point out that she kills you in a cutscene without giving you the time to act, so it's clear that there is no way to defeat her in combat.
In contrast to that good unwinnable encounter I've seen many of them implemented badly. The first encounter with Malak from Star Wars: Knights of the Old Republic is one. You can defeat him in fair combat, but the subsequent cutscene will still play out as if you lost to him. Another is Lothar (the skull man) from Planescape Torment. There is some talk that he is very powerful, but you're an immortal and when you encounter him he's holding one of your party members prisoner. It's only when you decide to fight him that you find out that he is powerful enough to kill even you. And how does he do it? I don't know; there is no cutscene or anything. When you attack him you get a message that you tested your immortality against the wrong being, immediately followed by the "game over" screen.
I think that if you're given the choice to respond to an encounter with combat then you should have a chance to win. That chance may be low (especially if there are other, more sensible responses) but it must not be nonexistent.