Everyone has few pet passwords they use in rotation in all the online presence. There’s a chance that after your password change on an application, the next time you might enter the old password due to inertia. When you do this on Facebook, it does a good job in reminding you with concrete details. Check out the informative error message:
This is 100% in accordance with the Jakob Nielsen’s Heuristic: Help users recognize, diagnose, and recover from errors. Well done, Facebook.