In the Max Lange puzzle I posted, any moves but the ones in the puzzle for black lose spectacularly, as far as I know. ..Qd5 is forced, as are all the rest. I stopped it short of the continuation because then you do have multiple options. It's extremely sharp for both sides though, because if you evaluate the position at the end, white has lots of extended pieces that can become misplaced. Sure, black has to find lots of moves to avoid immediate death, but if he does, he can equalize pretty easily and even gain some advantage.
In the online game I played in the Max Lange, I felt like I was finding only-moves for half the game, with any error being fatal. It was tons of fun =)
Here's the game, no analysis though. Maybe I'll annotate this sometime...


Interesting take on tactics training. I think some people assumed (including myself) that finding forcing lines over and over will ingrain them enough that you spot when your opponent has one available too... but I guess that's not always true.
Being able and willing to block a threat is certainly important. These defensive type puzzles are an interesting twist. I will say as you move on to harder tactical puzzles though it moves away from the one forcing line stuff and a big part of solving them is making sure you can meet all of your opponent's threats/defensive moves along the way.
If you don't see the threat in post #4 after a few minutes, then go ahead and look up the answer at ziryab's link. It's one of those that after you see it, you'll remember it pretty easily from then on.
Interesting. I've heard something similar. At the highest level, opening tricks and traps are less common, something like that. So, I would imagine that the threats would be far more subtle and sneaky.
Either way, a threat is a threat, no matter how obvious or subtle, and like you said, must be seen and dealt with. My goal is for these Threat Puzzles to be beneficial for all skill levels.
Enough talk, I'll post a few more Threat Puzzles after dinner, and continue to do so on a regular basis...until the cows come home.