Originally Posted by DrForester
Most suspicious thing about the episode was how Twilight talked about clues, not as something you need for proper investigations, but as something you better not leave if you want to successfully commit a crime. She totally ate some of the cake.
I'm convinced that honest Applejack got away with it. Twilight knows/suspects she was guilty, but she can't prove it, so she kept her suspicions to herself.
First, look at this scene where the ponies are tempted.
Twilight's the only normal-looking one. Applejack fits right in. Applejack was also the first pony to try and make an inappropriate grab for the cake.
Rainbow Dash stole a bite, but she got seen and left evidence.
Fluttershy stole a bite, but she got seen and left evidence.
Rarity stole a bite, but she got seen and left evidence.
Then Twilight pressed Pinkie to admit that she fell asleep on the job and there was a long window for a fourth someone to quietly steal a bite unobserved and leave no clue. Twilight wouldn't logically have pushed for that info if she had herself stolen a bite during that window.
And in this episode's inspiration, Murder on the Orient Express [major spoiler if you haven't read/seen it yet], everyone was guilty.
Twilight was the detective, and Pinkie was the client, so I can let them go. And then Pinkie went and ate the whole leftovers anyways.