Image has a point, Celestia. Why didn’t you know?

She don’t know grammer to.

Nah jk,

Another reason why she is useless.

Maybe she does know about them, but just didn’t include them in the list of possible threats because up to that point, they never attempted to take over a pony settlement by force, leave alone the capital of the country.
 
And even then, she wouldn’t know much about them, due to their secretive nature.

@Drasvin
 
He’ll even forget numbers.
 
…but he can’t forget he hates pears…

@SRD639  
While he has an excellent memory, even he forgets things sometimes. Usually with disastrous results.

@Background Human  
The Doctor says otherwise.

@archestereo  
Memory capacity and indexing would still be a problem. Sorting through a millennium’s worth of data at a moment’s notice probably isn’t easy.
 
Anyway, Celestia not knowing or remembering changelings is pretty forgivable. All the audience learns about them is that they don’t live exclusively in Equestria, and might not have been in the country at all previously.
 
Also, shape shifters. Very easy for them to hide.

They proooobably aren’t native

@Background Human
 
…but that’s because of the very deterioration that makes us mortal to begin with; if Celestia is immortal, then she isn’t going to have memory problems.
 
…actually, ya’ll do realize that the point ya’ll’re arguing isn’t the ONLY problem with that story, right? I mean, there’s like, alot; as in the only way to suspend your disbelief is to throw all critical thinking out the window.

@Background Human  
Strong with this one, the force is.

@Background Pony #8209  
Exactly! Who the hell thought it wad a good idea to attack a major city when it is in a period of especially high security rather than some small town with little to no security.

She’s probably seen ‘em before. But she’s been around for a while, and they say memory’s the first thing to go.
 
(When one thousand years old you reach, remember things as well you will not, hm?)

@MagnificentTrixie  
Cue ChrysaliaLover rage.

@Sjogre  
I’ve conceded to your point. It makes far more sense than theory I can come up with.

Wait, were they aware that the changelings were the threat to Canterlot? Like, they knew there was a threat, and they knew how to keep it out, so why didn’t they know what the threat was?  
And why didn’t the changelings just invade another town? I mean, not all the towns had giant force bubbles around them. Ponyville certainly didn’t.

@Sjogre  
Okay fine. Whatever.
 
I think maybe it could be one of those “Heard about them, but thinks they don’t exist” or something. But yeah, that still doesn’t quite work.
 
I’ll concede the point about her potentially not knowing anything about them. Still, she gives zero indication that she doesn’t know about them, but yeah.
 
Somehow, it just makes more sense that she is familiar with them, even though all plausible arguments say otherwise.

@Fortune  
…
 
Celestia not catching that Cadance was replaced requires Celestia having a low degree of familiarity to begin with. Celestia not catching that Cadance was replaced when she knows of shapeshifters is completely inexcusable.
 
And not checking the VIPs for replacement? Seriously? You start checking the top end and work your way down because infiltrators will do the most damage from positions of authority. That’s also faster than vetting all the grunts, simply because there are less people at the top.

Not really that farfetched. Changelings survive by using deception and trickery. At best, there would be folktales about them.

She probably hadn’t met Chrysalis before, but she knew what changelings are.

In retrospect it was sort of an oversight by the writers.

@archestereo  
Nope. She said Equestria. So, not only did the changelings spend time in other countries, but the wedding could very well have been their first mass move into Equestria, with no previous contact between ponies and changelings. At least, none that the ponies knew about.
 
Celestia has never been omniscient or even infallible. Celestia simply being unaware of the changelings is completely plausible, and doesn’t result in the logic holes that Celestia knowing about changelings but taking no steps to counter them creates.

She. . . She never said that. At all.
 
It would have made more sense to have mentioned the parasprites, but I personally think she was just saving Twilight from embarrassment with that statement as well.

@Sjogre  
Wouldn’t necessarily mean high on the list, but still potentially there.
 
She could have just not have suspected that they’d do something as overt as directly send their Queen in to replace a high ranking royal family member. She probably scoured the guards and servants and finding nothing there, dismissed the thoughts of Changelings.
 
Still kind of dropping the ball when it came to them, but not unreasonable. Besides, the whole point of doing it then was to tire her out and keep her busy through multitasking.