Note: What I say in the text below the quoted section applies to the context provided by the quoted section, and not to the out-of-context example provided by the OP.
Ourph said:
Acerak constructs a a dungeon headquarters for himself as a young, wannabe demi-lich. The "front rooms" (only a small portion of this dungeon) are constructed with the convenience of his servants in mind. He places no traps in these areas because his servants will need to move about freely to do their work and it's cheaper to buy new slaves than it is to construct effective traps anyway. However, at the entrance to his private rooms Acerak constructs a very expensive trap, it is very nearly undetectable and seriously deadly to everyone but him. He does so to prevent both his servants and his enemies from penetrating into his domicile (understanding the evil nature of his servants, he knows he must protect himself no matter how much he pays them or how many members of their family he holds hostage). He places it on a lever because he wants to kill only those who actually attempt to enter his area, not just anyone who enters the room. His servants know that if they require his attention they may enter the room and wait safely for him to appear, but shouldn't pull the lever. Everything beyond the secret door is filled with deadly traps that are tuned to leave Acerak and Acerak alone unharmed.
Eventually, Acerak shuffles off his mortal coil and takes up residence in his tomb as a demilich. His servants wander off and his HQ lies abandoned. Several generations of squatters make use of the front area of his dungeon (one displacing the next in various power struggles between humanoid tribes, evil cultists, etc.) but none have the means to get past his fiendish trap and enter his private sanctum.
Then the PCs happen along. They clear out the "safe" servants area of Acerak's abandoned dungeon. The portion which was constructed with no traps and, like many of the previous occupants before them, encounter the trapped lever and lose one of their number before discovering that the trapless nature of the rest of the dungeon ends in this room.
IMO, a perfectly reasonable and common setup for a D&D dungeon that fully explains the presence of a very deadly trap at one point of the dungeon even though the portion of the dungeon the PCs have already explored contained no traps or traps of a very different nature.
I've been thinking about this scenario some more. I will agree that it is a reasonable set-up for a dungeon, but there was something that was just bugging me about it. I've finally figured it out...
Under this scenario, a party could work their way carefully through the entirety of the 'public' parts of the dungeon. They carefully check every lever, every door, and anything else that looks suspicious, for traps, and find none. In every single case, this is because there is no trap.
And now they come to this room. They carefully search the room, and find the secret door, which wasn't that secret because the lever tipped us off. Still, a Search check was sufficient.
They then apply the Rogue's Search skill to the lever. They find no traps. Now, in EVERY PREVIOUS INSTANCE in this dungeon, this has been because there have been no traps. Suddenly, it's looking a lot more reasonable to assume that in this case there probably are no traps, isn't it?
Still, the party has a standard operating procedure with such things: the person with the best saves handles levers just in case. So, the same happens again here. And, despite having the best saves in the group, and having a really good roll, he still blows his save.
And, what's more, he is then insta-killed with no body, preventing an easy Raise Dead.
Basically, the DM has just inserted a MASSIVE jump in difficulty that has come out of nowhere (from the player's perspective), and which has had HUGE consequences.
And 'good play' can't even save you here. Unless the party has been extremely paranoid with every lever, every doorway, and every other thing that has looked suspicious, they have no reason to do so here. Do you really expect the party to use rope to pull every lever, to summon creatures to open every door, and to cast Augury every time the Rogue detects no traps?
It just reads like far too much of an intentional 'gotcha!' from the DM to be fair. And it certainly wouldn't be fun.