@EyeGuy  
The Royal Guards are mostly punchers, so they tend to see the world in terms of punchability; “thinking real hard” isn’t really part of their MOS.

To be fair in Twilight’s defense, she doesn’t live in the Crystal Empire so she wouldn’t think that’s Thorax’s disguise in the first place. I feel like that caption would be better suited for Cadance since she does live here.

@TexasUberAlles  
But the crystal ponies temporarily got suspicious that the Great and Honorable Spike the Brave and Glorious was a changeling.

You’re on the right track here, but the dropped ball was that she trusted Spike… to, y’know, be Spike. If that’s really him, she’d have no reason not to trust his choice in Crystal Pony friends, but it was kinda doofy to take it for granted that he wasn’t a Changeling leading in a co-conspirator.
 
Although… Spike seemed surprised that Thorax could turn into a Dragon and a rock, so it’s possible that Ponydom in general is unaware that Changelings can turn into non-Pony things, and she assumed he was a kind of mimic-proof failsafe.

@EyeGuy
 
I really enjoyed the ep as is, but i see and agree with your point.

@IceKitsune  
Yeah, it would have been smart.

For those confused about what I’m saying…
 
Nopony trusted Twilight or Starlight. They even temporarily distrusted Spike the Brave and Glorious. It doesn’t matter that he was an apparent pen pal to Spike, they still should’ve been at least a BIT suspicious of Crystal Hoof, because it’s been established changelings can shape shift into family and friends to feed off of your love.

@PonyPon  
Except as far as we know the only way they have to detect changelings is though knowing the pony and asking them something the Changeling wouldn’t know. She trusts Spike so likely thought that as Spike was his Pen Pal he would have done something to already check. It might be a weakness but there was nothing that she could have done to determine if he was one or not and it would be seriously OOC for Twilight to tell him to leave.

Yeah Spike pretty much vouched for him so there was no reason for Twilight to doubt him

At this point in the episode I kinda thought Spike would be like, “Twilight, I trust you - this is my changeling friend, we really need for help getting him to be accepted, what with you being the princess of friendship and all.”
 
But then that’s kinda what she did after the song, I guess.

Not to mention that even if you’ve seen them before it could still be a changeling.

@IceKitsune  
Because that degree of trust is a weakness in this circumstance.

Ummmmm Spike said that they had been pen pals and Twilight trusts Spike so I don’t know why she wouldn’t trust Crystal Hoof as well.