From the 10,000 word post:
"If I, as the player, throw a hammer at a closed box, intending to break it, and there is a magical device inherent in the box that causes it to explode upon impact, the player is rightly killed, though the trunk was revealed and the bomb was not - since detecting the bomb required more than the player's eyes. If, however, the player detected magic about the box, and was told there was no magic, and then caused the explosion, the DM is at fault for not giving COMPLETE information."
Yes, it can be difficult to know when you have, as a DM, given complete information. Very often your opinion will differ from that of your players. Very often, if you withhold information which your players considered complete, a confrontation will arise.
A DM must remember, first and foremost, that he or she is not perfect. He or she will forget to give information - something is overlooked, or forgotten, or simply not conceived. The party may be following the wheel tracks of a cart, and you overlooked describing the tracks of whatever is pulling the cart; or you may forget to mention that the cart is much bigger than an ordinary human cart - which might be important in determining its owner; or you may simply have failed to conceive that the cart, full of hay, will have left a trail of hay in its wake. Such things happen. There are invariably more details about a simple thing than you are bound to remember or describe. You can easily forget to mention that the box is magical.
In your lack of perfection, explain to your party that they have the right to appeal their situation, if it should happen that you made a mistake somewhere. If they think they're following orcs, and it turns out they're actually following trolls, fix it. Where it says above in the quote that the DM is at fault, this does not mean the fault cannot simply be repaired. Replace the trolls with orcs and keep the campaign going. Back the party up so they can make up their mind about whether they want to fight trolls. If you didn't give them the information they need, ADMIT IT. Dump your pride, address the situation and move on. The situation is not past resolution; you have absolute power at your fingertips to change any aspect of what's going on - so apply that absolute power and redress your wrongs.
On the other hand, if you DID describe something that was critical, and you DID give the information the party needed, and now the party is complaining because they weren't ready, don't give ground. Sometimes, a party will fuck up. Sometimes, a party won't pay attention to the right details and blunder into a situation for which they weren't prepared. This is not your fault. Unless your party can name for you the exact piece of information you should have given but did not, your party doesn't have a case, and you are free as judge & DM to rule against them.
Sometimes, a player won't accept such a rule. Sometimes, a player will see your universe-slash-world in terms of how they personally would run such a universe or world. This can be tricky. It usually results from that player having played in a different campaign where certain clues or descriptions were presented differently. Most often a DM will create a series of clues (short-hand) regarding monster strengths or combat expectations which become standard to that campaign. Players will often make the mistake of believing that such short-hands should be dogmatically applied to every campaign, including yours - and when they are not, there's the possibility of the player becoming irate and disruptive.
Do not let this dissuade you from your vision of how a campaign ought to be run, or what information players ought to be given. It is your campaign. I've explained that your players have a right to what their senses can tell them - you are not duty bound to label or mark your strongest monsters according to anyone's precepts except your own. If the ogres in your world have 20 hit dice, because that's the way YOU want it, your players will have to adapt. Of course, if your ogres have 20 hit dice, you might want to make them bigger - but you don't HAVE to. Many human high levels have 20 hit dice, and they're the same size, right?
In summation, then. Admit your errors. Fix them. Apologize to your party. Keep the game moving. And if you have a player that disputes your presentation upon preconceptions that originate from other games than your own, try to explain that to them. Try to make them understand that this is your world, and things can be different. Don't let the rest of the night bog down in a discussion of how D&D should be run, or how a DM should fit a particular mold of DMing. Settle the player down, or eject the player. Game night is not the best time for a philosophical debate.