One of the dumbest was the claim that the close fits of rocks in South America "so tight a knife won't fit in the cracks" must have been done by space alien technology. This silliness persisted until an archaeologist showed how to make such a joint in about half an hour by banging and rubbing rocks together.
What the ancients did do very well with, however, was manipulating the natural world to their benefit. I.e. making gardens, cultivating plants, managing livestock, etc.
Edit: for those who are wondering, the altitude and azimuth of the sun at the solstice can be accurately determined simply by erecting a vertical pointed stick, and having a bit of patience. That's it.
As to some of the more general science knowledge though, subsequent societies could certainly replicate the scale and precision of cities like Angkor Wat or Tikal. Likewise, later ocean navigators had far more sophisticated navigation methods, using such tools as the compass and the kamat. As to qanat and wind-catcher architecture, it wasn't ever really lost...
I'll grant you that knowledge of public health has been very uneven, though it has generally been on the upswing in the past couple centuries (as objectively measured by life expectancy and the burden of communicable diseases), nothwithstanding certain missteps (such as adopting droplet theory which was less accurate than miasma theory).