
Balkoth |
Say there's a 10 foot wide corridor and a mage fills it with the Grease spell. Then Orc #1 tries to run through and falls down. Then Orc #2 tries to run through (next to Orc #1) and also falls down. What happens when Orc #3 tries to run through? He has to pass through #1 or #2 but if he falls down...where does he fall? And what happens when Orcs #4-10 also try to run through?

Balkoth |
By Raw they still fall.
Where, though? All in the first square? So you could potentially have 9 orcs in the same square, all prone?
Now the dm might decide
I am the DM, I'm trying to decide how to handle this.
Also note it will probably only trip 19 orcs before somebody makes a spellcraft
Spellcraft is trained only :)

Loengrin |

3 methods :
- Random : 1d8 and Orc land in the square indicated by the dice.
- GM Fiat : You decide where logically for you the Orc should fall basxed on his movement, from how much he has failed etc.
- GM Fiat Fun : You decide the Orc fall in the square you think it would would be the most fun for the group... You can even pile them while describing how comically they look while falling... Slam them into wall (no damage or 1 Non Lethal) etc. :p
Edit : You can also use the Overrun rule since it's almost the same, someone try to go through a space and fail :
If your overrun attempt fails, you stop in the space directly in front of the opponent, or the nearest open space in front of the creature if there are other creatures occupying that space.

Obscure citations |

Accidentally Ending Movement in an Illegal Space: Sometimes a character ends its movement while moving through a space where it's not allowed to stop. When that happens, put your miniature in the last legal position you occupied, or the closest legal position, if there's a legal position that's closer.

The Mad Comrade |

Slippery stuff on floor makes us fall down is not a Spellcraft check, it's paying attention to the environment.
I would rule that the first 4 orcs are occupying the four squares of grease. Then it becomes difficult terrain until four more orcs are piled up, to which it is difficult terrain with a slope. After a dozen orcs are piled up, it is impassible terrain providing cover in both directions. ;)

Mykull |

". . . a layer {emphasis added} of slippery grease." The spell description doesn't say how thick that layer is, so it could be pretty thick.
You say that the orc is running through the grease and falls. I'd say Gorm the Gutspiller (Orc #1) slips and slides through the ten feet of greased area and five feet beyond that, ending up prone.
Ruk the Bonecrusher (Orc #2) slides next to the first one; also prone. Slag Sharptooth and Grib Backbreaker slide into Gorm and Ruk (Slag & Grib are now on the far end of the greased area (from the orc's POV).
Dab Deathmuncher and Murb Limpnoodle (don't ask) fall in the first two squares of grease.
Blurk Blooddrinker and Tink Flesheater can now move through the obstacle (X2 movement) over their fallen battle clan. Obstacles may, at the DM's discretion, require a check (I'd say Acrobatics: 15 because Dab and Murb aren't just laying their motionless.

Kileanna |

What about a harem of succubus trying to get through the corridor? That just sounds like a good time.
Careful! , they might start making grapple attempts!
I had a similar situation with cyclops in my S&S game. My players learned that they were kinda clumsy and lethal on melee thanks to auto crits so they started casting grease on narrow corridors and doors. It turned into a comedy.

Balkoth |
this would be more fun with goblins instead of orcs...
Goblins have higher acrobatics and are more likely to pass the DC10 check :P
At some point, I imagine the orcs will start trying to make DC 10 acrobatics checks to get over the grease area.
You mean trying to jump it? Since it's a DC10 acrobatics to move through it.
You're not supposed to be able to end your movement in another's square, so they would be pushed back or forward depending on the situation.
At some point they'd potentially trip in the grease and then get pushed forward 15 feet before they fall (or 15 feet back), was trying to see if that was intended.