Weird. you were walking along the bottom of the screen? Could you explain a little more in detail what was going on?
I went ahead and tried this, and apparantly it happens no matter what. It has to deal with checking for values outside of the array not being equal to -1. Well, I'll have to fix this little bug. I thought it was only in the bottom right corner, but I guess not. It shall be fixed later tomorrow (actually today, it's 1:05 here).
I know it's very easy right now. I am planning on making floor tiles in which you can only build in one direction - to limit the amount of power you have and to make the levels a little harder. I am also thinking about making a limit as to how many blocks you can just leave sitting around on the floor. There will also be some kind of teleporting system where you drop in one and fall out somewhere else. Evil tiles that kill you when you step on them, etc.
This is just a little demo I wrote in one day. How much do you expect!
Any other features I haven't mentioned that you might like to see? I don't even know how much further this little side-project will go, but it's nice to finally release something playable.