Isnt your terraform command button reddish, isntead of greyish, when its happening? I sometimes get my terraform button stuck as well, unnfortunately I can't recall under what circumstances... Always hitting ESC fixes everything tho...
I think it was when I was terraforming something with CTRL, SHIFT or ALT (one of thsoe, dont remember which) pushed (sth connected with ramps), and when I cancel the command during designing my terraform, and I press on ramp button again, without releasing the key Im holding even for a while.
Huj, is it the problem?