Okay, so I've been seeing a lot of this going around lately and I'm sick of it. I'm sick of both arguments on the subject. I'm sick of people harping on Pinkie for being blatantly rude to Flutters and I'm sick of how people are trying to defend Pinkie.
"Pinkie just made a mistake, she didn't mean it."
Yes, it was and no, she didn't mean to make Flutters cry, but it's more than just that.
"Well, she didn't prank Flutters in Griffin the Brush-Off because she understood how sensitive she was, the continuity is off and wrong and fuck you, you pink piece of shit."
No. The continuity is NOT off. It's actually spot on.
Think of it like this, in season 1 all the characters were just starting to get to know each other. Since Twilight was the reason they connected and became friends in the first place, none of them really talked to each other before she came along (as we can plainly see in Look Before You Sleep with Rari-Jack.) Pinkie didn't want to prank Flutters in Griffin the Brush-Off because she had an understanding that Flutters was fragile and scared. This being a new friendship, she doesn't want Flutters to think she's being rude by pranking her.
In Filli Vanilli however, they have been friends for a long time now and Pinkie feels more comfortable being herself around Fluttershy and the other mane 5. She over-exaggerates her friend's fears for hope that she'll see how foolish she's being. Pinkie is not just being stupid. She has a plan, her plan just backfired on her. At this point in their friendship, Pinkie thinks that this sort of behaviour will be well-received and acceptable. Being the extreme extrovert that she is, she doesn't understand Fluttershy's fears and thinks she just needs a good laugh to overcome them and make her feel better because that's how Pinkie deals with stress and fear.
Fluttershy on the other hand is an extreme introvert. The mere thought of crowds and people she doesn't know watching her on stage (or anywhere for that matter) terrifies her, petrifies her even. Rarity understands this fear better than any of the other ponies, probably because she's already overcome this fear (remember in Sonic Rainboom when she helps Rainbow Dash with stage fright?) Fluttershy doesn't understand that Pinkie is merely trying to make this fear humorous for her. Instead, Pinkie's words and most likely the imagery it brought on, made those fears more real than ever and sent her into a panic attack.
One thing a lot of bronies don't seem to understand is that Fluttershy isn't just shy or introverted or scared. She has an actual problem. Fluttershy suffers from Anxiety Disorder. It's something I myself have been struggling with for years now and it's not easy. She also has some signs of Social Anxiety disorder which doesn't mean you don't want friends, it just means it's harder for you to find and connect with people you like. You want to socialize and interact with people, but you can't handle even the slightest thought of being rejected by them so you mostly keep to yourself and stay quiet.
This episode is a fantastic character study between two characters who shouldn't even be friends in the first place, but somehow manage to be pretty close regardless. It's also a pretty accurate depiction of how someone with Social Anxiety Disorder functions in society. With a small, close group of friends and as little interaction with strangers as possible. This episode and a lot of episodes this season deal with very real, very serious problems and I personally love this season for it. This episode is basically Fluttershy's Flight to the Finish.