Having an eye for detail.
Trixie and Starlight were relieved at not having been caught out. All of the disruption they caused that day, from accidentally enchanting Bulk Biceps, Granny Smith and Jewel Stone to harbour a grudge against a certain 'Great And Powerful' mare, to depositing a massive piece of furniture in the middle of a spa to be used as a massage table, had gone unpunished. Twilight was none the wiser, and both unicorns were grateful for that.
It was as Trixie and Starlight were about to make themselves scarce from Twilight's 'it's not fair that we didn't break the puzzle record just because we had a little sing-a-long' lecture however, that things slightly changed on that front. The alicorn was just casually passing by the freshly-arrived-back table, when she stopped for a moment, examined it closely, before one of her eyes began to twitch erratically.
Noticing the princess's odd little tic as she was leaving with her friend, Trixie nudged Starlight ahead of her to point it out, and the two of them watched in concern as Twilight started to pull rather a disturbingly stressed face, the same kind of expression she wore while debating whether to send Starlight away or not a few days before... or, if you want to go further back, when she'd forgotten to send Celestia a friendship report for an entire week, shock horror.
Wondering what her problem could be, and worried about her favourite mentor, Starlight stepped forward in earnest to ask: "H-Hey there Twilight, a-are you okay? Y-You look kinda..."
"Crazy." Never one to beat around the bush, Trixie just spat out what was on her mind, as she perused the area to see if there was anything else worth transforming into teacups.
Starlight admonished her friend with an angry glare, before returning to Twilight. "What I was going to say is, you look as though something is on your mind. Care to talk about it now, teacher to student?"
Twilight's weirdly smirking head jerked up at this juncture, and she suddenly began to advance stealthily towards Trixie and Starlight in the same manner a predator might try to corner it's prey. "You want to talk? Fine, let's 'talk'."
The pair of unicorns began to reverse slowly, realising they might have made a mistake in enquiring as to the princess's troubles. "E-er, maybe l-later..." Starlight remarked, while beginning to look around anxiously for an escape route.
"No, now is as good a time as any, dontcha think?!" Suddenly, Twilight's horn flashed, and all three mares disappeared only to reappear seated at the large table, knocking over an unsuspecting Spike in the process who'd only wanted to find out if the Mane-iac was going to be caught in page 100 of his comic book.
"So, yeah. I was just passing the table a few moments ago..." the alicorn continued, almost stabbing at the furniture with her hoof. "When I noticed something a bit off. I decided to check again, and what do you know. I was right! Come on you two, you have some serious explaining to do!!"
"E-Explaining?!" Starlight had no idea how Twilight could've figured out what they'd been up to, so she decided to play innocent for now.
"Can Trixie have a lawyer present first, please?" Not wanting to take a chance with her freedom, the magician decided to unwittingly implicate herself.
Frowning at both of their inexplicit answers, Twilight decided the time for being subtle was over. "Explain... why this table is a full two inches away from where it's supposed to be! I measure its length in proportion to the room size every single morning, and clearly it's been repositioned without my permission! Not only that, but judging by the density of these grooves in the floor, it's been dropped from a medium-height onto the ground, not just knocked or moved, so don't deny it! You better have a good explanation for this outrage, Starlight... and accomplice!"
"Uh-oh..." Starlight realised then it was foolhardy indeed to expect that Miss OCD herself would miss any minor imperfection in her picture-perfect castle.
"Trixie doesn't want to die!!" Thoroughly intimidated by Twilight's belligerent demeanor, the magician hopped over from Fluttershy's seat where she'd been present to Starlight's lap on Rainbow Dash's chair, blubbing profusely while hugging her friend tightly for comfort.
Seeing the shaken two unicorns clinging onto each other, as well as noticing a disapproving shake of the head from Spike who was still knelt on the floor, Twilight saw that she might be overdoing things a tad. "O-Oops, sorry..." she stammered, somewhat embarrassed at her conduct. "I-I'll tell you what. Let's discuss what went on over a nice cup of coffee, shall we? But for now, there's something else I must deal with..."
Suddenly, in front of everypony present, a bright red mist seemed to gather above Twilight's horn, which was quickly sucked up and bottled away in one of her saddle bags. "That's that taken care of. I'll just put the kettle on..."
Trixie, about to ask if there were any tasty cinnamon nuts she could have with her drink, gawked in amazement at the princess's apparent method of dealing with frustration. "Hey, that's just like what you did with me earlier, isn't it Star?"
Starlight nodded in return, and decided to mention the coincidence to her mentor. "Hey Twilight! Whenever I get mad, I do exactly the same thing! Are you sure it's healthy, though? Because, today..."
She never got to finish that sentence however, as en route to the kitchen to prepare the brew, Twilight craned her neck around to listen to the conversation, completely missing one of Spike's old issues on the floor...
SLIP! "OUCH!" SHATTER.
The glass bottle in Twilight's bag abruptly smashed on her fall, almost immediately causing a terrible scarlet fog to coalesce in the air, before it formed into two separate plumes which swiftly headed towards Trixie and Spike.
Upon absorbing the colourful smoke, the pony and dragon's moods turned from confusion to anger, and they spun around with flashing eyes to face a nervous Starlight, all while yelling in very echoey voices.
"You hypnotised all of our friends!" shouted Trixie.
"You've never understood the basic concept of polished silverware as applied to friendship!" growled Spike.
"Why can't you make more reliable friends, like DJ Pon or Cranky Doodle?" Trixie insisted.
"Why do you change your manestyle so often? It's irritating!" Spike chastised.
"Listen to me more!"
"Eat everything on your plate!"
"Flush the toilet when you've used it!"
"Needs more cowbell."
"NNOOOO!!"
At this point, Starlight did what any normal, rational mare would do... and that's run like the clappers.
As her student was quickly followed in tow by her two possessed pursuers, Twilight wore rather a sheepish expression. "Hmm... maybe Celestias right. Perhaps I should talk about my feelings more in the future..."
Author's Note
Just a little sketch I wrote after the most recent episode, which I thoroughly enjoyed. 
Back to more serious writing soon... I hope! 
Oh, and Happy Easter all! 
