So I'm currently playing Indiana Jones and the Fate of Atlantis, and I'm stuck. I find that taking a small break and doing something else for a bit is usually enough to help me progress in those kinds of games, so I figured I'd ask you guys what the most difficult puzzles you've encountered in adventure games are!
Needless to say, this thread could potentially contain spoilers, but out of courtesy, let's try and keep it vague, eh? (If you have to include a spoiler, be nice and add a warning.)

Off the top of my head there are two puzzles I can remember:
The goat puzzle from Broken Sword 1.
The counterfeit ticket puzzle from the race track in Grim Fandango.

Those two caused major hair loss for me! So, what are the hardest puzzles you've encountered?

The final marble puzzle in Riven!

Or the number system in Riven.

I'll go with Riven. :p

This one didn't feel all that difficult once I figured it out, but I did spend three months trying to turn the power on in "The Dig".

I remember Riven inspiring me to create my own cypher based on a set of predetermined symbols. I think I still have the key around here somewhere. Yes...I am that much of a nerd LOL. :P

I always found either the poison cigarette puzzle in Under a Killing Moon (due to the timing to avoid being caught), or the a couple of the dead ends in Space Quest II particularly difficult. I am frankly surprised I ever made it through all 6 Space Quest games at all.

Ah, I just remembered another one - the hand sign puzzle in Monkey Island 2! I just could not figure those out.

The puzzle to get to the Selenitic Age in Myst. Figuring out what to do wasn't that difficult; it was actually doing it that gave me so much trouble. Trying to get the sliders to the right spot was infuriating, as it was far to easy to move the slider a bit too far without realizing it.

Also, I'm not really sure if it counts as a puzzle, but Monkey Combat from Escape from Monkey Island.

The worm maze in Botanicula - arghhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh!

But this inspired me to up my Google-fu and find a solution. I'll try it tomorrow and see if I can finally get past this puzzle. Otherwise I'm definitely making my son get through it for me next week when I see him ;-p

For anyone wanting the solution, here's the link - the solution is pretty far down the page so no real spoiler unless you actively scroll to see it.

http://amanita-design.net/forum/index.php?action=printpage;topic=3875.0

As to brain puzzles, The Myst series kept my brain in overdrive for months! I'm really looking forward to playing through them again at some point. ;-)

Ah yes, same here. An evil puzzle was the custard pie in King's Quest V. How can you move it too far without realising it? If you move it too far then the note is wrong so surely you'd just take it back a bit until the note is right again.

I can't help but remember the goat in Broken Sword 1

Nikopol has brutal puzzles. Even once I learned how I was supposed to solve them, I was still befuddled most of the time. One in particular involving getting a door open was near impossible. And the fact that you spent so long just to open an ordinary door was irritating.

If the topic isn't restricted to adventure games, then Silent Hill 2 and Silent Hill 3 have some incredibly hard puzzles if the puzzle difficulty is set on hard. For example, either of them had one where you had this poem about a monster devouring a woman, piece by piece, and you somehow had to make the connection between the words and a numpad (the poem reveals the order in which to press the buttons). Or one with books, that require you to decipher Shakespearean texts in order to get a code. Or that's how I remember the puzzles at least, it's been a while and my memory's hazy.

The Longest Journey's Rubber Ducky puzzle comes to mind easily. Like seriously Funcom, how high do you have to be to make a puzzle like that and KEEP IT IN THE GAME.

Discworld, hands down.

Pretty much any puzzle, actually. My favorite is, however, putting a frog in the mouth of your drunken past-self. Yes, you can meet yourself in the past and you are drunk because in the present you drank counterwise wine which makes you drunk in the past. Where was I... oh, right! You put a frog into your past-self mouth to stop him snoring so that your present-self who traveled to the past can catch a butterfly. The only hint for that puzzle is that at the very beginning of the game you get hit by a spell and spit out a frog. And how come you have a frog in your bowels? Well, isn't that obvious? You have to feed you drunken past-self, otherwise it wouldn't make sense at all! But then you can ask yourself one more question: where did this frog come from? Your present self has it because you spit it out because you already had it in your bowels because your present self put it in the mouth of your past self, and obviously he did it when he traveled to the past but since he brought it from his own present, this frog didn't exist in the past so you couldn't spit it out in the first place and hence you couldn't put it into your mouth. But you did anyway! Gah!!!

Oh, and don't worry if you've never played this game and it doesn't make any sense to you. I have play it many times and it still doesn't make much sense :) Don't get me wrong though, I still absolutely love this game :D http://www.youtube.com/watch?v=Ro2GfpglLlc&t=0m59s

The Frogwares' Sherlock Holmes games have some difficult ones, but a lot of easy ones too. Newer the game, easier the puzzles :)

And the already mentioned Longest Journey rubber ducky thingie.. Didn't come to mind to try that exactly.

Probably the hardest for me would be some time-relevant puzzle, meaning you had to be at the right time at the right place in order to solve the puzzle. I can't give any specific examples right now, but I seem to recall something like that in e.g. Darkseed?

The microscope puzzle in The 7th Guest was hard, but in a good way. It wasn't really an adventure game puzzle, but a bit like a tic-tac-toe or checkers minigame or such. Quite hard, but the more I played it, the better I came with it, eventually beating the computer opponent.

The "hardest" puzzle in a clever way I recall now was the monkey wrench puzzle in Monkey Island 2? Ok it was a mere wordplay, but it got stuck into my mind and I was fully DOH! after it.