If you’ve been GMing for any length of time then you’ve probably allowed your characters to have something you soon regretted. Sometimes it’s handing a low-level character the +5 Holy Avenger, sometimes it’s letting the investigative psychic have the mind-reading power, and sometimes it’s letting the military characters acquire a lance of the most powerful mecha on the battlefield.
When such things happen, it can be difficult to “take it back” without hurting feelings or impacting the campaign. In some cases, the rules work against you; the PC’s new power was a legitimate XP purchase, the uber-weapon was lying in a treasure trove of a published adventure, or the war machines were legitimate salvage.
I think in theory, most GMs would say “if it is ruining the fun, get rid of it,” but this is easier said than done. Is it worth the aggravation and hurt feelings? Is it easier to try and work around it, even if it means cutting your campaign much shorter than you’d intended?
I’ve found that, as I’ve grown with experience, I can catch these things early and nip them in the bud with minimal fuss. The player is usually content so long as she doesn’t feel weakened by the taking; she gets her XP back, she’s offered something else of equivalent, but less game-breaking, value, or all the PCs take the same hit, equalizing the loss.
There are still times, though, when a player gets to keep something for several adventures before I realize it’s an issue (or I’ve stopped trying to work around it, which also isn’t fair to the player). By this point, the player has become attached to the new power and she has a paper trail of adventures to back her argument. Unless she can see how her power made those adventures less fun, then she’s going to have a difficult time wondering why you think it’s a problem.
So, fair or foul? If a PC acquires something that is problematic, should you take it away? Do the circumstances of the acquisition or the timing of the proposed taking matter? If you did take something away, how did it affect the rest of your campaign?