Jenny100 has already mentioned a couple of my pet peeves. I worked like a dog to figure out the mandela or snake puzzle in Beyond Time, and then discovered that I had one of the "right" solutions that wouldn't work!

And I hate any slider, but that one in Timelapse, where you had about a 25% chance of getting a conformation that couldn't be solved was the worst.

Not to mention the puzzle where you had to move the white squares at the end of Reah--even with a printed solution in front of me, it took about ten tries; there were so many steps that I kept losing my place.

Judging by the number of saved game requests we get on Hints, I would add the laser/robot puzzle in Still Life to that list too...

The Mandala problem in "Beyond Time" (which I wrote a computer program to solve) is well worth mentioning in this thread.

There's going to be some individual differences here. If one is colour-blind to some extent problems involving colours are difficult. Others (and I count myself here) have issues with musical or aural puzzles.

Puzzles I feel are unfair in Adventure games are those which are still difficult even when you know the answer. A case in point is the Spaceship music puzzle in "Myst" - the solution was easy, applying it was pain pain pain. Or for me the last puzzle in Obsidian - I knew what one had to do but there was no way I could see and remember how all the pieces moved.