Be wise after the event
Definition 1 of 1
Pro Tip 1/3Hindsight Bias
This phrase is used when someone realizes the right thing to do only after a situation has happened. 
After the stock market dropped, many investors claimed they knew it would happen, but that's just being wise after the event.
Pro Tip 2/3Regret Implication
Using this phrase can imply regret for not making a better decision earlier. 
We locked the doors too late and the thief was already in, a case of being wise after the event.
Pro Tip 3/3Unforeseen Consequences
It emphasizes learning from unforeseen consequences or outcomes. 
I didn't take an umbrella and got soaked; I'm always wise after the event.