The rules are what I say they are. Loopholes only matter to software and machines, not men.

Let's get more specific.

Let's say you're playing D&D 3.5. A player creates a 1st level Psychic Warrior. For purposes of this scenario, let's say that you've decided to allow psionics in your campaign, but that you have the power to adjudicate any ambiguities in the rules.

Now, this psychic warrior has taken the Call Weaponry power at 1st level. Here is the power's rules:
http://www.d20srd.org/srd/psionic/powers/callWeaponry.htm

How do you rule in the following situations?
1. The psychic warrior calls an elvencraft longbow (from Races of the Wild) to their hand.
2. The psychic warrior calls a splash weapon, such as a flask of holy water, to their hand.
3. The psychic warrior calls an improvised weapon, such as a lantern, to their hand.
4. The psychic warrior calls a siege weapon, such as a ballista, to their hand.
5. The psychic warrior calls a monk's unarmed strike to their hand.

1.  Is he/she an elf? If no, then no.
2.  Maybe, might end up calling an open bucket of oil, and splashing it all over your self.  
3. Is he proficient with improvised weapons?  If no, the nope. If yes the sure you get a random improvised weapon.  Probably a bar stool.
4.  Too big, you don't have the strenth to call that.  Or squish.
5.  Either nope.  Or yep.  A psychic warrior imbuing himself with psychic energy to do damage makes sense to me.

More importantly, if they try all that in one session its time for a chat.  Maybe they just have a new tool and are trying to find out what it does.  Cool, I think that is creative constructive role playing.  Lets talk about that power and work out what it does, what its limits are, and then we will both know.

Maybe he's trying to pull a fast one.  Well maybe I don't want to play with him any more, or maybe I just expain that its only a first level spell and can't do all that.  Since its a psychic power I can add some psychobabble explination if I really want to.

"Oh, right, that's why I never run 3rd edition. Okay, gang, show of hands...5e or 1e?"

Calling a siege weapon to their hand?

"As a shadow overhead blots out the sun, your last living thought is a deep fellow-feeling for Wile E. Coyote."

Quote from: Cave Bear;1008239Let's get more specific.

Let's say you're playing D&D 3.5. A player creates a 1st level Psychic Warrior. For purposes of this scenario, let's say that you've decided to allow psionics in your campaign, but that you have the power to adjudicate any ambiguities in the rules.

Now, this psychic warrior has taken the Call Weaponry power at 1st level. Here is the power's rules:
http://www.d20srd.org/srd/psionic/powers/callWeaponry.htm

How do you rule in the following situations?
1. The psychic warrior calls an elvencraft longbow (from Races of the Wild) to their hand.

I never allow splats sight unseen in my game. If I'm allowing this splat, though, this would be ok.

Quote2. The psychic warrior calls a splash weapon, such as a flask of holy water, to their hand.

Holy water is under "equipment," not "weapons."

Quote3. The psychic warrior calls an improvised weapon, such as a lantern, to their hand.

No. If it's not the "weapons" list, the player can piss right off. If Call Weaponry could be used like this, it would be called Call Object.

Quote4. The psychic warrior calls a siege weapon, such as a ballista, to their hand.

Siege engines aren't "weapons" in the D&D sense. Your weapon proficiencies don't apply to them, you don't make attack rolls with them, the usual rules for Full Attack and so on don't apply, etc. When a spell or ability in 3.5 says "weapon," it doesn't mean "any old thing you can think of that does damage."

Quote5. The psychic warrior calls a monk's unarmed strike to their hand.

An unarmed strike is an attack, not a weapon.

Quote from: Cave Bear;1008239Now, this psychic warrior has taken the Call Weaponry power at 1st level. Here is the power's rules:
http://www.d20srd.org/srd/psionic/powers/callWeaponry.htm

How do you rule in the following situations?
1. The psychic warrior calls an elvencraft longbow (from Races of the Wild) to their hand.
2. The psychic warrior calls a splash weapon, such as a flask of holy water, to their hand.
3. The psychic warrior calls an improvised weapon, such as a lantern, to their hand.
4. The psychic warrior calls a siege weapon, such as a ballista, to their hand.
5. The psychic warrior calls a monk's unarmed strike to their hand.

1. Fine, your bow is made by elves. If you want a +1 bow it takes 4 more PP.
2. Not a weapon.
3. Not a weapon.*
4. Not a weapon, for the purposes of the power.
5. Not a weapon.

* After thinking about it a little bit, I might bend on this. Depending on the thematic nature of the power, if it was appropriate for a lantern to be a weapon in that instance, I would allow it. Say, fighting a mummy or some other flammable critter.

Honestly, if a player tried to argue he could summon a monk's punch to his hand, he's one of those contentious jackasses that ruins RPGs and probably needs to be run off from the game.

5. OK, you summon a Monk's punch to your hand.

Quote from: bryce0lynch;1008125Someone shows up at your OP game with a character that is infinitely powerful. They found a rule loophole, and it seems legal.

What would you do?

Tell them my game's not going to be a good fit for that style of play.

I'd like to hear what the OP was thinking of when he talked about legal but infinitely (which I am taking as hyperbolic) powerful exploit. Cave Bear's doesn't work in 3e, but let's say it did. In that case, that is simply an instance of the rules not being so unbelievably lawyerly/legal-contract level specific* to exclude an overpowered use. I have never found such exploits to be a real problem in my games, because player's are pretty good about not pressing the issue when DM's say either "that's an utterly-ridiculous interpretation of that rule,' or, 'if I were to allow that, it would break the game open, and then I'd have to bring something equally as stupid online to contest that build. We're not going down that road.'
*and even that's a misnomer, because 'no reasonable individual would interpret that line of text in that fashion is a valid legal argument

What tends to actually be a problem more in games (IMO) are usually more subtle broken builds that are simply overly good (not great, not infinite power, just a little too good), or break a benefit/opportunity-cost curve in some way. 3e's perfect example might be clerics getting all sorts of niceties in exchange for having to be the party healing-battery, but then being readily easily able to hand that responsibility off to low cost wands of curing.

As to advise to give, or what I'd do about it. I tend to just give a brief speech at the beginning when DMing new people about how we are all adults trying to have fun playing a game, not here to break the game over our knee and make everyone else want to play something else. "Don't be a jerk" is a surprisingly effective phrase.

Either there are no loopholes to exploit in the games we play or my players aren't asshole who would try that crap because they aren't trying to best the game or prove some point. I also don't allow someone to just show up with an unusual character he's concocted without talking to me about it in advance.

The question is why the hell are you playing a game that makes it possible for a player to  manipulate the rules to become infinitely powerful?

Quote from: Cave Bear;1008239Let's get more specific.

Let's say you're playing D&D 3.5. A player creates a 1st level Psychic Warrior. For purposes of this scenario, let's say that you've decided to allow psionics in your campaign, but that you have the power to adjudicate any ambiguities in the rules.

Now, this psychic warrior has taken the Call Weaponry power at 1st level. Here is the power's rules:
http://www.d20srd.org/srd/psionic/powers/callWeaponry.htm

How do you rule in the following situations?
1. The psychic warrior calls an elvencraft longbow (from Races of the Wild) to their hand.
2. The psychic warrior calls a splash weapon, such as a flask of holy water, to their hand.
3. The psychic warrior calls an improvised weapon, such as a lantern, to their hand.
4. The psychic warrior calls a siege weapon, such as a ballista, to their hand.
5. The psychic warrior calls a monk's unarmed strike to their hand.

1. Legitimate. Unless the item does not exist within the campaign setting.
2. Not a weapon, fails.
3. Not a weapon, fails.
4. Not a weapon usable by a warrior in single combat, fails.
5. Not a weapon, a skill, fails.

That one would be pretty easy to adjudicate.