That's Not Sally

Robotnik captured the real Sally and made a robot duplicate to find Knothole. Even though the duplicate was good on imitating her, its inability to be a mother figure to Tails clued him in that this was not the Sally he knew. Tails told Sonic that it was not Sally, but Sonic thought Tails had a nightmare. Later, Sonic does the secret handshake with her and didn't correct him when he turned the thumbs up to a thumbs down, exposing her as a fake.

Example of:
Imposter Forgot One Detail