Twilight, Bark Wing, and Pandora were preparing for their next crusade 6 months after Twilight had arrived. This time the attempt for Twilight's cutie mark was shield surfing, Pandora created the track, while Bark Wing used her control over wood to make a wooden shield. Twilight's job was to use the shield to surf down the track, which was basically a tall mountain with a bunch of obstacles in a single path from top to bottom.
Meanwhile
The Cutie Mark Crusaders were flying through the air in a large pile at a fast speed heading straight for Canterlot Castle after another failure at a cutie mark attempt.
In Canterlot Castle
In a room in Canterlot Castle where a mirror was hidden. One Celestia had been planning on moving to the Crystal Empire when it returned, only for it to still be under the control of King Sombra, with the barrier keeping him and his army out, while the former element bearers now without elements after they disappeared returned to Ponyville unable to do anything. Celestia herself stood looking at the mirror in sadness.
"Why do I keep failing as a teacher." Celestia said. "First Sunset Shimmer runs away through the mirror to another world. Then Twilight completely disappears, and we can't find her, after the wedding fiasco I wouldn't blame her if she ran away as well. The only good thing is that the mirror is still closed, but we still don't know where she is."
With a heartbroken sigh Celestia leaves the room, and closes the door, only for a few minutes later the mirror to start to glow, as unknown to her, Celestia's words had activated a hidden function of the mirror, to find ponies lost on other worlds, but only if they did not get there through the mirror in the first place. The mirror had just finished and showed filly pegasus Twilight shield surfing when a living ball made up of cutie mark crusaders flew through the open window, bounced off the floor and through the mirror, with it changing back to normal right after.
With Twilight
Twilight knew she was in trouble when the shield broke right after she jumped over the tree sap lake in the middle of the path. And with their problems with tree sap, this is a bad idea. But before she could either finish that thought, or spread her wings to fly to safety, a portal opened above her, and dropped three fillies Twilight knew well onto her back causing all four to crash into the tree sap lake, with bark Wing and Pandora a looking amused from the sidelines, at least until the ground gave way, and they fell into the sap as well.
When they climbed out they were covered in tree sap, with Twilight, and the crusaders first out.
"This was one of our biggest failure yet." One of the crusaders said.
"You said it." Another replied.
"Ah just wish, I knew how being launched through a window and into a mirror sent us here." The third said.
'Wait those voices.' Twilight thought. "Apple Bloom, Sweetie Bell, Scootaloo, is that you?"
The crusaders snapped to look at the pile of tree sap the size of them, and in the shape of a pegasus, recognizing the voice as well. "Twilight? What happened to you?"