In all but the most sandboxriffic of games, GMs put things into play specifically for certain players and their PCs. In many cases, this might be as simple as polling players on what sorts of enemies they want to face - or creating in-game rewards that will have special meaning to one or more of the PCs. This is, in my mind, a good thing.
Like all good things, though, it can be taken too far... In extreme cases, this ends up with a Monty Haul campaign. We don't need to go to extremes in order for such practices to become problematic, though.
What happens when a GM introduces something which is - in his mind - for a specific character, but that character isn't the only one interested in it? If it isn't something sharable - and the GM isn't scrupulously fair about who gets it - there are going to be problems.
I'm being vague here. To a point, this is intentional, as the concern here is a general one. Things introduced by the GM don't have to be physical objects - they could be in-game business/social opportunities, in-game relationships (friendship, romantic, and/or sexual... or even rivalries!), or any of a wide variety of other things.
When the GM does this repeatedly, there are issues of favoritism at play.
How can this be less of a concern?
Option 1: Call attention to it. If the GM notes beforehand that "this subplot-or-whatever is for Player X," other players won't get their hopes up. Also, repeated toss-outs to one player will be more obvious.
Option 2: Use rules. Some White Wolf games have rules for spending XP for new backgrounds. In D&D you could easily institute an XP cost for all magic items (base it off the creation costs).
A related issue is when a single player (or PC) lays claim to things in the game that don't really belong to them any more than to other PCs. Many will say that this is a PC-problem, but I have mixed feelings on that. Sometimes it is totally a PC problem. When Sparrow claimed a recovered First-Age city as her own domain in Nick's old Exalted game, I didn't have a problem with it - it totally fit within the themes of the game... but I've met players who bank on feelings of 'party unity' and the reluctance of other players to get into conflicts between PCs in order to get a larger share of group resources than they have any right to...
Any thoughts on how to handle this sort of thing?