I haven't watched the video yet but I definitely will! These kind of mistakes make me really question myself because even though I have watched these episodes so many times, there's always something I haven't noticed!!

Maybe I've been focusing on the plot so much and been complaining mostly about the plot holes
But I haven't noticed there were two Piffs in the scene you just mentioned
It's actually so nostalgic to see someone listing the mistakes because as a kid I read so many Winx Club blogs and people used to list the mistakes there too. So this topic is like a trip back to my childhood
I remember that in the Black Mud Swamp episode of the 1st season Flora's shirt kept changing its colour from green to red and vice versa. In the Finnish dub there's also a mistake: in one scene the whole audio stops and Musa's voice is gone even though she should be talking. There're overall so many mistakes in the Finnish dub where the wrong voice actor says the lines of the wrong character.
In Musa's concert episode "Show must go on" in the 2nd season some specialists were simultaneously performing on the stage AND in were the audience too.
The colour of Layla's Enchantix outfit changed from blue to green during the 3rd season.
The same secondary characters are seen as students in the auditorium of both Alfea AND Cloudtower.
Flora's magic Winx outfit was changing its shape and was missing some fabric in the 1st episode of the 3rd season.

Professour DuFour has been casually walking in the Gardenia and she was also accompaning Musa's mother with piano when she was singing in the Show must go on episode. I don't know if this is an actual mistake but a fun fact anyway since it feels like she's
everywhere.

I've seen some other secondary students in Gardenia too during the different seasons, also some of the secondary witches! Gardenia seems like a popular place for the magical creatures to go on a holiday, then!
Also Faragonda and Griselda were in the audience of Miss Magix but nevertheless it seems like Stella didn't get caught on participating to the competition and skipping the school.