Hindsight . . . 20/20
Didn't see this listed and thought it should be on this website.
I think most people know what it means, however.
I believe the phrase is more often "Hindsight is always 20-20."
I've seen that version (bob's) and others. People say, for example, "And with 20/20 hindsight, we realized . . ." It isn't so much a set phrase as a set idea. The wording varies.