DProject: If the topic isn't restricted to adventure games, (...)
Nah, any puzzle you've had trouble with in any game is fine. Although I'd argue that the Silent Hill games could easily be described as adventure games, or certainly as games heavily loaded with adventure elements. :)
timppu: 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?
Ah, it's not quite the same, but I absolutely HATE timed puzzles in classic adventure games. You know, the ones where if you don't figure out what to do within a certain amount of time, you die? I hate those. Not because the puzzles themselves suck, but because dying usually means having to spend another five minutes watching the unskippable cutscene that often happens beforehand.
koima57: I think the key word here is logic, it must fit naturally within the context.. If it doesn't, it is fake difficulty to me, and if it does, it is good design.
I agree, if the logic behind a puzzle isn't logical, so to speak, the game quickly winds up in my dustbin.
There is a puzzle in Professor Layton and the Curious Village where you are provided a board with pegs on it and asked to draw seven squares that can not share corners. I eventually had to look it up, and I got rather annoyed when I realized that while the puzzle asked for squares, (and the Professor Layton-games are usually extremely precise in their wording,) it actually allowed for squares that weren't angled at precisely 90 degrees.