In what sense are they a mistake?

Certainly not WRT Hubble, Chandra, GRO, COBE, WMAP, Gravity B, Pioneer, Voyager, Magellan, Ulysees, Viking, Mariner, Galileo, MGS, Mars Odyssey, MRO, Mars Observer, Mars Pathfinder, the MERS , SOHO, ACE, Cassini, NEAR, Lunar Prospector, AND the multitude of Earth observers.

WRT manned spaceflight, one could make the case but nevertheless, here we are a bit wiser on how to proceed from here. I wasn't there when the decision was made away from Apollo so I don't know what was known and why that decision.

I could point out that space systems were extremely primitive by todays standards (the computers on Apollo missions having less ability than a wristwatch) Advances in electronics and computing has made more functional missions possible now.

Yes my friends, I feel your pain. I see a thousand points of light. Each point, a shining city in a crater. We're ready for another go at the moon now, scientifically and technologically. Astronomy, Cosmology, experimental physics (direct access to the largest roughing pump in the universe), a lunar seismic net, the sky isn't the limit anymore. We're going to launch rockets from Florida. Then, we're going to launch rockets from California, and Iowa, and Montana, and the Pacific and Kasakstan and China. And then, we're putting up a space elevator for CATS so we can all go... WEEEEEEEE-HAAAAAAGGGGHHHH!!!!!