Flatter to deceive
Definition 1 of 1
Pro Tip 1/3Initial Impressions
The phrase often relates to something that seems promising at first but fails to meet expectations later on. 
Her polite manner flattered to deceive; she was actually quite rude.
Pro Tip 2/3Hidden Truths
It suggests that the truth is hidden behind a seemingly attractive appearance or promising start. 
The sunny morning flattered to deceive, and by noon it was raining heavily.
Pro Tip 3/3Cautious Optimism
Use it to express skepticism, advising someone not to be too optimistic based on initial success. 
He scored in his debut, but those numbers flatter to deceive; let's see his season performance.