In the 3rd Edition rewrite of the famous module White Plume Mountain, the titular dungeon gets modified highly successfully to use 3rd Edition rules while staying very true to the original version in style. One specific encounter, however, sticks out as a very poor conversion…
Early on, the group is given a crossroads of three directions to choose from to tackle in any order. Almost immediately down one of these paths (canonically the "first" one you're expected to do, if the order it is written has anything to say about it), you encounter a slime trap. The rules state no way to avoid or defend yourself against this trap – simply that green slime falls from the ceiling when an adventurer passes below it, automatically hits them, and deals terrible damage (including Con Damage) continuously unless removed or burned promptly. It will immediately kill most typical adventurers in only a few rounds.
This particular trap offers no saving throw, opposed attack roll, use of trap-finding, or even a spot check. This is not the case for every single other element of the dungeon, which is full of Reflex-Save or Attack roll traps and ambushes that are all labeled and given statistics.
What happened here? Why is this trap so special and hardcore – and one of the very first ones in the dungeon, no less? Reviewing the 2nd Edition version, the trap was similar there (just even more likely to instant kill, what with Save vs Death), but all the other traps were similarly more hardcore, and were revised appropriately in 3rd Edition.
The rules for Green Slime are actually not unique to this dungeon, and are available at the SRD under Dungeons. It says that they hit a 5-foot square and deal their damage and effect there, but no rules are given for any kind of defense. This seems to be a major contradiction in mechanics and style to everything else in 3rd Edition.
Alternatively, are there any fair house rules for this situation? A reasonable spot check difficulty or reflex save ruling for how to avoid this slime?
Clarification: The major facet of the question is the inconsistency of the rules between this trap and all others in the module. 2nd Edition situations are well known for their difficulty, and 3rd Edition for toning down that same difficulty. The fact that this one type of obstacle is the only one that seems effectively "unconverted" in nature is what is being called into question.
Best Answer
As you say, this is how Green Slime is presented in the DMG (3.5e p77, 3.0 p117). Note that Green Slime is considered a hazard, much like a flow of lava, raging river, or 40' pit would be a hazard.
Green slime is green slime. Does the module say the slime is invisible? That the players can't see it? Is there an illusory wall or darkness spell concealing the ceiling? Exactly what is stopping the players from looking around and seeing that the walls and/or ceiling of this room are entirely covered in thick, wet gobs of outrageously neon green slime? Green slime is meant to attack characters that go running down corridors heedless of obstacles or without light sources, or to restrict their movements. Without magical aid or PC stupidity, it's not subtle. Even if the module says "the PCs don't immediately notice" the slime, that doesn't mean it's sequestered. If the PCs say "I'll check the area for traps" or "is there anything on the floor or ceiling?" then they're going to see it. Sometimes you have to read between the lines.
Also, remember that green slime responds to movement—any movement that might be big enough to be food. It need not be a PC. It need not even be alive. Green slime is not cunning. It doesn't wait for the right time to strike. It drops on the first thing of reasonable size that moves underneath it, so it's unlikely that more than one PC would be affected. It would not wait for the whole party to be below it to drop.
Next, most DMs I've played with rule that the slime has to burn through the armor before it deals damage to PCs. The rules aren't clear, but they do imply that this is what happens. Combining that with the fact that it states that fire destroys it (such as a torch) and it can be scraped off during the first round (such as with a torch or dagger), and you see why it's only CR 4 (the same as an 80' pit, or roughly a 20' spiked pit covered with an illusory wall). Green slime is literally defeated by uttering the phrase "I scrape it off with my dagger". This means that, IMX, in almost every instance a player has been affected by green slime the results have been that the PC's armor and weapon have been damaged or destroyed... and that's about it.
Truly, the difficulty of green slime is getting past it once you know it's there. It's still deadly, but you need to find a way to get a lot of fire on the ceiling (burning hands or produce flame) or get it to drop for you to burn ("what did we do with that orc's body?") or find a way around.