There are a couple of Source wells that are on grassland hexes, but could be mistaken for path due to the graphical treatment - such as those on Isles 2 and 4. As a general rule, Source wells will not appear on path hexes, as that would make it possible for that hex to be the only valid path connecting to a fortress, and then for that path to be eliminated by a spire being built there.
If you own the first printing of Cloudspire, there are a few hexes that have unclear terrain. On more recent printings, these hexes have been altered to be more graphically clear.
On Isle 7, the hex that looks like this is meant to be plains:
On Isle 8, the hex that looks like this is also meant to be plains: 