Sometimes when you move the mouse around the level area without clicking you get white outline boxes wherever you move the mouse to.
http://img87.imageshack.us/img87/5015/screenshotk.jpg
Bug in level editor (3.2.4)
Moderators: Flumminator, Zomis
This happens to me sometimes too, but I haven't found any pattern to it. 
(This bug was originally reported here.)

(This bug was originally reported here.)
The H. World levelset can be downloaded from http://www.bd-fans.com/RnD.html -- search The H. World on that page.