On April Foal's Day, an aristocrat demanded an audience with Princess Celestia, despite Celestia having promised to spend the day with the filly Rainbow Dash. Celestia compromised. The aristocrat got his audience, and immediately began ranting on and on about a minor problem. It was five minutes before he realized that the pony sitting on the throne was Rainbow, not Celestia. Celestia had, however, been magically projecting her voice through Rainbow. Celestia explained she had been cursed to assume the body of a filly. The aristocrat asked if there was anything he could do to help, and Celestia informed him that the only way to relieve the curse was to pass it on to somepony having a worse day than her. The aristocrat immediately insisted that, despite his complaining, that he was having a great day, and hastily excused himself.
In May The Best Friends Win, Trixie is trying to leave Ponyville after getting tricked into revealing her nasty(er) side to Rainbow Dash. Dash keeps using her Super Speed to pop up in front of Trixie before she can get far, though, and clearly enjoys toying with Trixie.
Princess Luna unveils the Royal Canterlot Voice to Rainbow, who imagines the princesses using that to shout from the bathroom for toilet paper, or in the middle of a more "nocturnal" activity, which leads her to "pushing that thought to the back of her mind, digging a hole, burying it, constructing a house to hide the hole and then throwing the loudest party imaginable in the house in order to obscure any possibility of said thought resurfacing."
In the first chapter of Hot Heads, Cold Hearts, and Nerves of Steel, Dinky, on Cloud Kicker's advice, sets Rainbow Dash up for a Spit Take. "Are you and Mommy banging?"
In the fourth chapter of Hot Heads, Cold Hearts, and Nerves of Steel, Shining Armor is unable to recall the lock combination for the safe containing the Elements of Harmony. A minor irritant becomes a disproportionately large problem when it becomes a matter or pride:
Cadance took a step forward. “Do you want me to—”
“No, thank you Princess, I’ve got this.” Shining grunted as he started spinning the lock, seemingly at random.
“Are you sure? Because I know the—”
“It’s under control.”
“It. Is. Under. Control.” Shining asserted, a mad gleam in his eye. His pride as Captain of the Guard was on the line now. He would not fail! Especially in front of his sister and the princess!
When the others come back into the room a few minutes later, Shining is smacking the safe on the floor in an effort to open it. And then Applejack is able to open it by lightly smacking the hinge, to Shining's shock.
At least half of the conversation between Dinky, Pip and Alula while they're trying to escape from the mine tunnels under the Crystal Empire, especially when they're arguing over whether it's boys or girls who have cooties.
According to Pip, she also once set some applesauce on fire as well.
Cadance, on a dare, belching the Equestrian national anthem.
Pinkie Pie says she can do the same from "the other end". The others are very insistent that she not demonstrate.
All the mares ganging up on Shining Armor to give him a makeover.
The bushwoolies' argument over whether stew or quiche is better, and the foals encouraging it in order to distract them.
During the Mane Six's fight with the bushwoolies, Twilight points out how their "Dark King" (Sombra) should be calling himself Emperor if he's ruling over the Crystal Empire, and also how the "Empire" is more of a city-state. The Chief bushwoolie ends up begging her to shut up, while Applejack is telling her that maybe she should actually fight them (Twilight refuses, saying proper terminology is important).
When Rainbow Dash is experiencing Sombra's memories of his duel with Celestia, she tries to distract herself from feeling his pain by focusing on her own happy memories, like Celestia taking her to the opera as a bonding experience... and then decides that the duel is actually less painful than the opera.
Bear in mind, Celestia was repeatedly kicking Sombra in the crotch, and Dash still thought that was less painful than the opera.
After sparking a food fight, Rainbow begins observing her friends for unusual behavior to see who might have been replaced by a changeling. When she sees Pinkie leap out of her seat in literal slow motion shouting “Nooooooooooooo…” to swallow a thrown roll with her mouth, and then moves at normal speed after swallowing it, what is Rainbow's response? "...Nope, nothing strange here."