I give these modules a lot of grief for not making sense. Thunderspire has more than its fair share of contrived plots, half-baked schemes and unlikely scenarios.
But at the end of the day, if the game is fun, nobody cares whether it makes sense.
That's a philosophy that's fuelled many classic D&D dungeons. It's led to magnetic ceilings, antigravity rooms, frictionless corridors, and such ridiculous-but-wonderful creatures as the rust monster, mimic, and gelatinous cube. To a large extent it's the core of traditional D&D - finding bizarre solutions to improbably deadly conundrums.
It's not something that either Keep on the Shadowfell or Thunderspire Labyrinth have embraced until now. They've hovered in the middle, presenting encounters that are ludicrous, yet not fun. The Proving Grounds, though, is where Thunderspire finds its old-school form.
This encounter is the Hall of Enforced Introspection, one of the tests of Baphomet that players must overcome as a precursor to facing the Guardian. It's an L-shaped room that players enter from the south, with an altar in the northwest that holds the Face of Baphomet. The Face is a cursed mask, one of the four items the players need to complete the Proving Grounds.
The catch is that the room is littered with columns, and each column is plated with mirrors. The mirrors are magical, and have a variety of effects. At the start of each player's turn they are "attacked" by a mirror; a successful "attack" means they've glanced into the mirror and are subject to its effects.
The mirrors nearest the entryway and the altar are teleportation mirrors; looking into one teleports you to its twin. This means that characters can travel straight from the doorway to the altar and seize the mask - but there's two catches. The first is that the altar is guarded by a pair of vicious Boneshard Skeletons, more than capable of ripping apart any adventurer unlucky enough to encounter them without backup. The second is that, of course, the mirrors are two-way, and the character who teleported to the altar on his first turn will be yanked back to the entrance again on his second. (This makes for a cruel surprise for characters who navigate the mirrors the hard way, only to be sent back to the start just as they reach their goal.)
The second variety of mirror is a more traditional trap. It's called a "draining mirror" and simply does a big dose of necrotic damage to its unlucky victims. The players, by the way, can avoid all these mirrors just by closing their eyes - although that leaves them blinded and offering combat advantage to the Boneshards, which has its own problems.
It's the third type of mirror which makes the encounter memorable - while at the same time being its biggest weakness. The "trapping mirror" transports anyone it "hits" to a demiplane known as the Oubliette of the Empty Mind. It's a small room with no exits; there's no way to get out from the inside. The room's only feature is a gnoll, who got trapped here when Maldrick tackled the tests and is now starving and half-mad.
Being trapped in a confined space with a hungry gnoll sounds like an exciting scenario on paper but in practice it's deeply dull. One-on-one combats don't work well in 4th Edition, largely because neither combatant has any real reason to move. The fight boils down to a series of flavourless attack and damage rolls and it's hard to hide the fact that the gnoll is only here as busywork for adventurers luckless enough to get trapped.
The real problem is if three or more of the party get hit by the trapping mirrors. Without enough heroes remaining active in the "real world", defeating the Boneshards can be extremely difficult (especially given they detonate with a damaging area-of-effect attack when bloodied, and again when killed). The only way to free those trapped is by triggering an indentation on the altar, which isn't easy while locked in combat with the undead. Unlucky rolls can make this encounter end with dead PCs and some or all of the party trapped forever in an extradimensional prison.
(The module, to its credit, suggests that if the PCs do become completely trapped, Maldrick's gnolls may eventually release them to interrogate them. By that time the captives will be dead and the Proving Grounds will be rendered moot. It's an ugly solution that undermines Thunderspire's few remaining strengths.)
The awkwardness of the Oubliette aside, this is an encounter that players seem to love. Mine had a blast - in fact they liked it more than I did - and in trawling the web for play write-ups it's one of the most commonly described (and enjoyed) encounters in the module.
Bearing in mind that this encounter works, so I don't really care:
 How did Maldrick's gnolls beat this room without killing the Boneshards? Does the magic of the Proving Grounds regularly resurrect the skeletons, perhaps?
 It's a DC 15 Perception check to figure out how to use the altar to free those trapped in the Oubliette. Was Maldrick honestly so callous that he couldn't be bothered to press a button in order to save one of his troops?
 Not actually a question - but for those following along at home who are wondering why the holy items aren't with Maldrick, the module explains that they teleport back to their "home" rooms after being used to summon the Guardian. Convenient.