Don't ignore someone's bad behavior that occurs in private just because they make your family member happy.
Don't just frantically accuse someone without sufficient proof.
Alternate Aesop Interpretation: Had Cadance not been revealed as an impostor in the next episode, a case could be made for the Aesop being "just because you're stressed about your wedding, it does not excuse you treating everyone else like dirt."
There is a lot of leeway regarding how much of Shining Armor's hostility to Twilight can be attributed to his brainwashing.
Did Twilight's friends, mentor, and brother really abandon and shun her after her outburst until her vindication, or did they walk away (possibly for only a few minutes) to give everyone time to cool down, then fail to find her again? One of Equestria's most talented unicorns couldn't have been kidnapped, especially not in the middle of a secure castle on high alert, so she must have left on her own. Worrisome, but there's no reason to think she's in danger and they can all have a long talk after the wedding.
Base Breaker: Cadence being an alicorn has had mixed reception, in part due to how this interferes with previous interpretations of Celestia and Luna. It doesn't help that, in this two-parter at least, her being an alicorn is pointless to the story.
Even before The Hub's premiere, a few fans called Executive Meddling on this two-parter, since it concerns a frilly, girlish wedding of two characters only introduced this episode that no one has ever heard of before for the sake of Hasbro's marketing, while other bronies called it a fitting way to end the second season.
The debate over Twilight's confrontation with the others likely would have caused the episodes to be Overshadowed by Controversy if not for the action-packed payoff of Part 2. Many feel the others were in the wrong for dismissing her in spite of everything she had done by that point in the series to show she wouldn't make such an accusation baselessly, knowing Cadance well enough so that her suspicions should have been taken seriously, and their not giving Twilight a chance to explain herself more rationally. Others Defend them arguing Twilight has been known to Freak Out before, she wasn't acting in a way that lived down her accusations of jealousy, and she failed to give any evidence that couldn't be rationalized away (like Cadance forgetting their Secret Hand Shake), giving them enough to justify their dismissal. There's also those that think both sides were equally right/wrong and/or want the others to get over it, since, in the long run, it's a minor plot point compared to the others that happened in the two-parter and the rest of the series.
Foe Yay: The Incest Yay Shipping for Twilight Sparkle and Shining Armor gets more shippy after the latter calls the heroine out for saying that Cadence is evil. Watch that scene as many times as you like. It would soon start sounding like a Sibling Rivalry conversation.
Fourth Wall Myopia: The rest of the Mane Six turing on Twilight can fall into this since, unlike audiences, they didn't get to see Twilight's flashbacks that would prove how suspiciously out of character Cadance was acting. Also, the most obvious sign (different colored magic) has never been verified to be observable in-universe.
Cadance is ironic because of Hasbro's pink Celestia's toys.
Awhile back, there was a "Smug Cadence" meme which had a picture of the bride-to-be with smug face making snarky comments about her being another alicorn, her being pink, and her marrying Twilight's brother among other things. First episode airs and Cadance is shown to be very smug and snarky. In truth, the imposter was and the real deal isn't.
Twilight when she straight out accuses Princess Cadence that's she evil during the wedding rehearsal. What was she expecting?
Why Twilight's friends, family and even Celestia, of all ponies, had suddenly forgotten that Cadence had been acting like a jerk to them, and was Not Herself today, especially after Twilight says that she is evil. This might be mitigated by the whole "getting stressed preparing the wedding" thing.
The Woobie: Twilight. Dear Celestia, Twilight! Not only did her brother ditch her, but so did her friends and mentor... and then she turned out to be right!