Crystal palace is a highly rated team, and so the result shouldn't be surprising to anyone, Chelsea won the Europa conference league and the club world cup whereas crystal palace won FA cup beating Manchester City and Liverpool such result is not a common one, though I believe Chelsea could have done better against crystal palace but based on the recent form of both teams I think the result was fair enough.
Chelsea did terrible job when playing with Crystal Palace that made surprise by suprirse from beating Liverpool in Community Shield to Chelsea in Premier League. Chelsea know that losing a match does not mean losing a battle and they still have many big battles to play and win big titles from Premier League to Champions League.
They know that they must come back too good form in Premier League and prepare for many difficult matches in Champions League. A defeat by Crystal Palace but Chelsea can consider it as an accident while they must learn from it in order to avoid similar bad matches in future.