You can get stuck in clear-terrain mode if you hit 'c' and then fail to point the character in a direction with blocked terrain. This can be exited by hitting escape, but it's not obvious what is going on.
Mediated by making clean-terrain the default when touching terrain.
Next step should be to create a reliable "prompt" that lets the user know what state he/she is presently in.
This seems to be a non-issue as of 0.8.