I know everyone loves my Traps posts. So here goes.

This is my most hated trap for any RPG. The lava stream runs across the hallway. It's only 3-4 feet across - an easy jump for any non-hobbit. So a player jumps the gap, hits the wall, and falls into the lava. In one game, the dwarf had another player toss him across. He bounced into the lava.

Best of all, depending on how you define "invisible", Detect Magic might not even work. Could be a clear crystal wall in a dark hallway.

But Sandy! No saving throws? Just instant death? You monster!

Saving throws are lame. I want my players to use their brains. They could toss something across the lava. They could levitate across instead of jumping. They could use Spot Hidden and notice a reflection in the crystal wall.

Should I give them a hint? Isn't it a big enough hint that I have an easily-jumped river of lava in the hall? Obviously something's not kosher.