Plot Oversight in PS/SS?

[Derek Hiemforth]

No: HPFGUIDX 138856

I've been re-reading the whole series recently (since finishing
HBP), and a thought struck me.  Please let me know if this has
occurred to anyone else, or if I'm just overlooking something
very obvious.

In PS/SS, the professors set up the protections for the Stone
at the beginning of the year.  Hagrid obtained the stone
before the start of term, and, by the opening feast,
Dumbledore warns the students about the third-floor area,
so Fluffy must already be up there.  Hagrid provides Fluffy,
Sprout the Devil's Snare, Flitwick the flying keys, McGonagall
the  chessboard, Snape the potions puzzle...

...and Dumbledore the Mirror of Erised.

So if the Mirror of Erised has been guarding the Stone since
the start of the year, down behind a gauntlet of traps, how is
Harry able to stumble across it around Christmas time in an
empty classroom?

Am I missing something?

(And on a lesser, possibly related, note... if the trap door
Fluffy guards is on the third floor, then when the trio go
through it, shouldn't they just end up on the second floor
instead of deep below the castle?)

- Derek