Twelve Level-less Problems

In games with character levels, I'm intrigued by dangers that persist regardless of where a character is in the track of advancement. (Opponents in head-to-head combat are almost never scaling problems. Fighting a knot of eunuchs is impossible at first level, easier at fourth.) When a scenario features level-less problems, it becomes interesting to a wider variety of parties, and is more likely to encourage creative play.

1. Instant Death effects. Lava. bottomless pits. trains that don't need tracks. wind-up telegraphed strikes.
2. Riddles
3. Convincing strange people to be reasonable
4. Hard decisions between two values
5. Shadow version of you that has all your powers(!)
6. Object must be retrieved from the world of dreams
7. Object must be retrieved from the world of ghosts
8. Oh no! You're in charge of a group of irresponsible weirdos
9. You are cursed. Probably curable, and high-level characters might have an easier time compensating but the story is about suffering, which both should be able to do in equal measure.
10. Extra effect against the higher leveled. Reopening all your previous scars. Burning every spell in your skull. summoning the ghosts of everyone you've killed. 
11. Attribute damage
12. Literally just ask your players to come up with the things that would challenge their PCs, then incorporate those things without changing them.