I don't know if anything related to this changed with .85 or .86, but all of a sudden I'm noticing it when I didn't on .75 or .77.
Tiles that look like plains are actually hills or forests. The only way to figure this out is to mouse over them. In the following picture, the tile to the northwest of my pioneers is a forest tile, even though it looks like a plains tile. You can see in the upper right that the tile is forest:

In my last game I observed the same thing with tiles that looked like plains actually being hills.