Once upon a time Rarity offered to help Twilight with one of her experiments, her curiosity piqued by some of the things that Spike had told her about. She handled her role as scribe fairly well up until the last part of the spell, when an unfortunately-timed sneeze summoned not a delicate flower but a strange green vine-ridden plant. Rarity reacted about as one would expect... sufficient to say, she no longer offers to help with any of Twilight's experiments.
Coincidence I think. The feather was just for writing, and I would've had to figure out how to draw smoke in if incense were involved... (I'm pretty lazy. XD )
Coincidence I think. The feather was just for writing, and I would've had to figure out how to draw