Reasons that have been provided thus far
1. You mistakenly listed the item in a room you didn't intend it to be in
Fine - mistakes happen. Here you've a choice to make: eat the mistake and leave it where it is, or put it where it should have been. In my view the general cut-off would be at the point the PCs get to the complex/dungeon/adventure; after that you're locked in.
2. During the course of play you decide it will make a better session for it to be a different room - do I also need to walk you through possible reasons you might think mid session that the treasure should be in a different room - or are you good coming up with a few things there on your own?
Doesn't matter, as any such reasons won't and can't stand up to:
There is the very old school more gygaxian style of gaming where you set up a dungeon and see if your players can beat it and act as an impartial referee as they go though it.
...which is, if the game's to maintain any integrity at all, how a DM has to be: an impartial referee.
I don't find that style of gaming to be very prevelant these days
Allow me to put on my curmudgeon's hat for a moment here.
That this style of play isn't as common can be put down to one root cause: player entitlement. This is something Gygax quite rightly railed against: players should have no built-in expectation that things will go smoothly, nor that they won't be pounding their heads in frustration sometimes, nor that their PCs will succeed at their tasks or even survive the attempt. That way, when they do succeed it actually means something.
And that player entitlement has found its way into game design - it's harder to kill PCs, other bad things have been mitigated (e.g. disintegrate just does h.p. damage) or removed (level drain, most item loss), levels advance at the speed of light - all of which make the game easier to play and at the same time much less challenging, much less engaging, and thus much less enjoyable in the long run.
Oh, and you don't have to get off my lawn but while you're on it the mower's over there...
