Two “soft retcons” for Season 1.
Celestia miscalculated the date of Nightmare Moon’s release - Twilight was meant to have a whole year to make friends in Ponyville. That she succeeded without that amazed the Princess, who had thankfully informed few enough ponies it wasn’t too hard to keep her error a secret.
In turn, Twilight shortly afterwards somehow misread a letter of notification about a local dragon problem as meaning she and her friends should do something about it, rather than sit tight and wait for the professionals. Celestia went over the text of her letter several times, but in the end concluded that the missing words must have been in Twilight’s brain… although her tricking her pupil into a “mission” before likely hadn’t helped.