If you deconstruct the same cliff twice it will try to blow it up twice too.
Also if a cliff is already blown up it'll try to blow up nothing.

Thanks for the report, both of these problems are now fixed in 0.0.4.

The cliff explosive ghost will stay even if its cliff is destroyed, but the robot will not use the explosive when it arrives. I've done it this way because there is currently no event triggered when cliffs are destroyed (they don't count as being mined or killed). I could just keep checking there is still a cliff underneath all of them, but that seems unnecessarily resource intensive.

So what will happen in this case? Will the robot fly back with the explosive? Or will the explosive be dropped?

The robot keeps the explosive and flies back with it.