Usually, I can trust my players; the only problem in D&D that can occur is one of alignment (e.g., one LE or NE character, one CN character, one CG character, and one LG character

). But then, D&D isn't my most commonly played system by a long shot.
That said...
The only time when things get problematic is when my resident freak/near-munchkin and I aren't "on the same page" regarding what a certain game is to be like - especially when playing a point-based RPG (like GURPS, which I kind of "opened" by removing the limit of points in disadvantages a character can take).
'cause in that case, he often creates characters with disadvantages that are so extreme that allowing him to play the character for even the five minutes it would take him to self-destruct would be more than enough to severely threaten the other players' more sensible characters' continued well-being.
The solution to this problem, of course, is discussing the problem with him directly and honestly. Sometimes, he just needs a few more points to make the character workable (in which case I just give him - along with everyone else - some additional points to help him make the character unproblematic, but not necessarily enough points for everything he wants); sometimes, he just went overboard with the character concept (which usually means that he created a character that has mental and/or social disadvantages that, in combination, are so problematic that none of the other PCs can work with him
at all).
Heh. I guess that main problem that I have with this player is that I tend towards less "cinematic" games, while he's so used to wise-cracking, butt-kicking, movie-quoting games in which the PC party works together for the sole reason that they are, well, the PC party, and NPCs have so little "personality" that they can't react in ways that weren't intended by the GM when the PCs get "creative" (and the GM can't come up with even remotely "realistic" consequences even if the players, say, use small nukes to get rid of their opposition), that he often reflexively creates characters that are just incompatible with my (and my other players') outlook.
Heh. But it's not like I'm always dead-set on introspective campaigns that die when exposed to too much movie-quoting and mindless mayhem; I recently bought both GURPS Cliffhangers (which is all about pulp adventure in the vein of Indiana Jones or whatever) and the Feng Shui RPG (which is all about simulating Hong Kong action movies) - with which he should have an easier time creating characters that he likes
and that are actually usable within the parameters of the game...

That's assuming, of course, that he
doesn't create any of the following (or similarly problematic) GURPS Cliffhangers characters: mad - with a capital 'M' - scientist, unscrupulous archeologist or "adventurer" who kills everyone who gets in his way and works for the highest bidder even if that is a dictator or leader of an evil cult, etc.
