In r5016 on level 7, we had several wall obstacles with the y coord 47 which was
the map length for the y axis of the map at the same time.

One wasn't able to remove these obstacles with the editor because they were
unselectable.

I removed them manually, but we should make sure this bug doesn't occur ever again.

Perhaps the walls were normally planted at valid positions and the map got
cropped afterwards, I don't know.

You can actually remove these walls but the disco mode doesn't work.
I think there is the same issue on the x axis.

Samuel, this is for you.

This would be nice to have done by for 0.15, but not essential.

See:
http://sourceforge.net/mailarchive/forum.php?thread_name=20111104081105.402361b0%40freedroid.org&forum_name=freedroid-discussion