First Things First
While it might be hilarious to poison the PCs to death and reincarnate them as fuzzy animals, I urge against it. In my experience, such events are seldom as funny at the table as they were when imagined. I suggest instead a supernatural poison (perhaps inhaled) that as its damage inflicts an effect like baleful polymorph. This is much easier to remedy, probably won't have any long-term campaign impact, and is just as hilarious.
A Fair DM Can't Make Traps Undetectable...
Following Pathfinder's guidelines for traps means that an adventuring party willing to commit the resources (e.g. creaturepower, effort, money, spells, time) will at least find if not avoid all of a dungeon's traps. There is literally nothing a fair DM can do if dealing with PCs who have decided not to fall for printed traps. Bonuses to the skills Disable Device and Perception will make such traps trivial exercises in die rolling... if dice are even rolled, given that in many situations the party can just take 20 on the Perception skill check and take 10 on the Disable Device skill check.
(A DM can eliminate a lot of the PCs arbitrary bonuses by plunging the room into deeper darkness and other unusual circumstances, breaking their toys, or forcing the PCs to do things on a clock. Often, however, only this last doesn't feel punitive unless handled carefully.)
...But a DM Can Be Fair about Being Unfair
The DM can provide multiple warnings about the potential unfairness of his upcoming traps. The DM can...
- allow the PCs to hear rumors of the horrors that have befallen those who've entered the dungeon.
- have the PCs meet crippled survivors of the dungeon who tell them of its nastiness.
- leave abandoned adventuring camps (complete with starving mules) near the dungeon entrance to which no adventurer has returned, perhaps with makeshift maps of the first few rooms, warning of hazards.
- deposit the corpses of those who chose poorly near vicious traps.
- inconvenience, maim, and murder henchmen, hirelings, summoned creatures, and flocks of sheep first--before the PCs--to demonstrate traps' horrors.
- allow monsters in other dungeon rooms to warn the PCs of upcoming dangers, perhaps in desperate attempts to get the adventurers to spare their lives.
Then the DM can spring upon his players a trap requiring an arbitrarily high Perception skill check to spot and a subsequently arbitrarily high Disable Device skill check to disarm. The DCs for mechanical traps are set by the DM, and after the DC is higher than 29 it doesn't increase the CR of the trap any further to set the DC at 30 than it does to set the DC at infinity. So set it to infinity. Trap goes undetected, it's sprung, and hilarity ensues.
Thing is, when given these sorts of warnings, some PCs will wisely heed them. I hope there's an alternative adventure planned for when they do. I also hope that--if someone encounters a DC infinity trap and survives--he has my favorite Pathfinder magic item, the trap-stealer's rod.