‘The Face of the Great Green Devil’ from S1 – Tomb of Horrors is perhaps the most famous trap in D&D history. And with good reason: it has everything a good trap needs.
1. It’s deadly: who leaps into the mouth of the devil is completely and forever destroyed.
2. It’s easy to detect: any PCs taking the time to examine the devil’s mouth will certainly detect the sphere of annihilation almost immediately.
3. Examining it is still dangerous: the PC examining the trap shouldn’t put his hand in. This is why you have a 10-foot pole.
The 2E module Return to the Tomb of Horrors improves the trap, and makes it even better, in my opinion. In that scenario, anyone stepping into the devil’s mouth with dust from Acererak’s discarded physical form in hand will not be destroyed, but is transported to The City That Waits.
Tomb of Horrors is often cited by new school players as the foremost example of an “unfair” module. Although it certainly was designed to be deadly, examining everything, having a thief to scout, and detecting magic or evil regularly saves a lot of trouble. It seems to me in new school games, the most common way to interact with traps is falling in them. Instead of teaching players not to walk into traps, traps became less and less dangerous.