Moving over hidden characters

[Sammael]

What happens when someone moves into a square where there is a hidden character without realizing it?

The reason I'm asking is because the following situation occured tonight in a combat practice round (outside of the real campaign, thankfully): the party's halfling rogue was successfully hidden in a forest square (with undergrowth). The party's fighter was looking for the rogue, and walked into the square (without having spotted the halfling). I declared that the fighter automatically spotted the halfling (who had a ranged weapon ready and thus couldn't get an AoO). The rogue's player accused me of screwing his character up and asked to see where this was stated in the rules. I called "realism" (a bad move on my part, admittedly), which he rebuted with "why can't I attack the fighter's unarmored head, then, if it's realism you want?"

Any suggestions?

Additionally, how would this rule relate to the situation?

Opponent: You can?t move through a square occupied by an opponent, unless the opponent is helpless. You can move through a square occupied by a helpless opponent without penalty. (Some creatures, particularly very large ones, may present an obstacle even when helpless. In such cases, each square you move through counts as 2 squares.)

 

[Abisashi]

First, I'd have allowed the rogue to still attempt to not be seen, but given the fighter a big bous (+10, maybe?). If the rogue succeeded, I'd allow them to occupy the same square as long as neither one was engaged in combat (if that happened, the fighter would probably step on the rogue as he manuevered in the square.)

What was the rogue doing? If he had done something on his last turn, I'd not allow the following. If he was just chilling, he should have been delaying his action, though if he didn't I'd let him retroactively do this anyway. Note that this should actualy require a readied action to do:
-The rogue could choose to stop delaying and shoot the fighter as the fighter attempted to enter his square; the fighter would be flat-footed against this attack, and wouldn't recieve an AoO because he wasn't aware of the rogue. The fighter would know where the rogue is at this point.

If the rogue did not make the aforementioned ranged attack, I'd allow him to make an AoO with his bow as an improvised weapon. The bow takes an amount of damage equal to the amount it deals, except on a crit, where it takes damage as if it had only been a normal attack.

 

[Abisashi]

Thinking about it some more, the rogue really should have readied an action. So, ignore my middle paragraph. I'd still allow him to make an AoO with the bow. It should probably do 1d4 + str damage.

 

[Zappo]

If the fighter was fighting, running, or doing anything where he has to move around a lot, I'd rule that he'd just bump into the hidden character with no way to avoid it without being noticed. Otherwise, he may make a new Spot check, and the rogue must also Move Silently versus his Listen, since he certainly has to move out of the way as the fighter moves in the square.

The realism argument doesn't hold water, there are many degrees of realism. In any case, the rules say that "you can't move through a square occupied by an opponent", so technically the fighter should just be unable to enter the square, i.e. bump into the rogue. I think that your call was perfectly reasonable even if I would have done differently.

 

[Sammael]

Thanks for the replies. I'm considering giving the person who moves into the square a +10 on his spot check if the rogue remains hidden and lets him enter the square (if the rogue threatens the square, he gets an AoO against the character before he gets to spot the rogue if he wishes). If the person is actively searching the square (spending his action to do so), he gets a +20 bonus on the check instead. If the person performs any sort of vigorous movement (such as running through the square or fighting another creature while standing in it) without having noticed the rogue, the rogue must make a Tumble check, followed by a Move Silently check, to avoid being seen. How does that sound?

Making the bow an improvised melee weapon (doing 1d3 damage for shortbow or 1d4 damage for longbow, for example) sounds fine, although I'm not sure I'd allow sneak attack to work with an improvised weapon.