What can cause brush loss?

[lamefun]

I'm making a spike pit with a lot of spikes. However when I compile it, some spikes are not displayed, however, they still block player movement. What could cause this?

[Plague Bringer]

Duplicate brushes.

[lamefun]

Duplicate brushes? Why do they cause this? I made this pit by Ctrl+C/Ctrl+V'ing an initial spike btw.

[Plague Bringer]

Duplicate overlapping brushes. The spike that are missing have two or more brushes in the exact same place. Delete them. I don't quite know why it happens, but it happens without fail.