The point is that you can never be sure whether you are or are not operating under the Misdirection spell cast by someone else when you cast detect "whatever" on yourself... and thus it is not infallible.
Sorry, but, while Misdirection would affect a "Detect Evil", it has no effect on Know Alignment, since it only affects spells that detect auras. ... That being said, isn't there a Know Alignment spell in 3e? I just checked the SRD, and couldn't find one. Was it removed since 2e? Jeez, learn something new every day.
But, also, since spells like Misdirection only affect detection spells, there are easy ways around that. Place Evil Bane on the crown (after all, who wants an evil king) and every day the king puts his crown on. If his head doesn't melt, then he's still good. An Evil Bane Pin, prick the finger of the king and know for sure. Heck, Glyph of Warding can be set to Good alignment triggers. If the king puts on the crown, and his head explodes, guess he wasn't good anymore, and I hope the king's days are short since Non-detection doesn't last that long.
IOW, for everything you could try to make the test fallible, there are a dozen ways to get around it.
But, again, this is really not my point.
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Caveat: The following is purely my own opinion and ONLY APPLIES to me, Hussar. It is not meant to apply to anyone else. Only me. I hope this makes it perfectly clear that I am only speaking for myself and not any greater or broader truth. All that lies after this point is solely the opinion of Hussar
((I hate that I have to put that caveat on there, but, sheesh, I've been accused like three times so far of trying to claim badwrongfun when that is not my point.))
For me, D&D is a game of Heroic Fantasy. It says so right on the tin. And D&D does this extremely well. If I want to do Conan, or Tolkien, or Cook's Black Company, D&D would be my go to game. OTOH, there are things that I don't feel D&D does really well. If I wanted to run a court intrigue game, for example, I would not use D&D. Say the PC wants to influence the court in order to pressure the king to do X. D&D's skill system is too simplistic for my purposes, and the magic system is too pervasive. Trying to abstract D&D's skill system across several weeks of effort just doesn't work for me. 4e comes close with its skill challenge system, but, again, that's really clunky compared to other games out there. For another example, if I wanted to do Call of Cthulu, with its descent into madness and PC's that will inevitably always fail and fall (hopefully in spectacularly interesting ways) I would not use D&D. The trajectory of a D&D character is opposite to what I want to happen to a Call of Cthulu character. CoC characters don't get more powerful as they progress, they get weaker and usually a lot deader.
Which rolls me back to Planescape. The idea of using an RPG to explore morality is an interesting one to me. I did it a couple of years back using another system called Sufficiently Advanced. I think it succeeded. Although it did get a bit silly at the end with the PC's saving the galaxy through the use of a very stretchy invisible whale scrotum. Sigh. So much for serious toned gaming.
But, again, Sufficiently Advanced, for me, is a much better system for something like this. Using the above example of someone convincing a group of people to change their beliefs and thus causing the area to shift to another plane, would be, again IMO, better handled by a skill resolution system that can scale between minutes, hours, days, months and even years. There are games out there that do have systems that can do this.
Am I saying that you cannot do it in D&D? No, of course not. Obviously that's not true since lots of people like Planescape and it does what they want it to do. For me, it doesn't. I would be fighting the system every step of the way, either as a player or a DM. To me, D&D doesn't come with that particular toolset. There are other games that do. As I've stated already, I would be interested in the Planescape setting if it was ported into another system that wasn't designed for heroic fantasy.