We have 2 unmarked areas of level 23.
5. Magical Repository
"... Dome. A deep, 20-foot-high alcove in the north wall contains four invisible stalkers trapped inside an invisible, dome-shaped wall of force rising from the floor. (Any items that Halaster removed from the mine carts in level 15, area 1, lie on the floor inside the dome. The dome appears to contain nothing else.)..."
Said space is not marked. On the screenshot I have included marked the space that in fact is noted in the map in the book version. This should be noted on the GM layer so that only the GM knows exactly where this dome of force is.

6a. Lightning Pillars
"...Creatures standing outside the hall are safe, as are creatures within the 20-foot-square space marked on map 23..."
Said space is not marked on map 23. On the screenshot I have included I changed the map maker "6A" to mark the safe spot. This notation should be made on the GM layer as you don't want your players to know where the safe spot is.

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