When you move an item over a path, it turns red to signify that you can't place the object there. If you cancel the move while the path is red, it stays red until you enter the move tool again.

Steps to reproduce:
Start any map
Build some paths
Place a building
Select the move tool
Click the building you just made
Move it over top of a path to make the path turn red
Cancel the move (by right clicking)

Video of bug: