The HUD textures are mostly alpha transparent and you can select them without knowing it. I have done this with trees before and I just now turned the controls on the Mystitool I was wearing into unreadable flat black circles. Fortunately it was the free copyable version, but now I have to go through the trouble of launching a fresh copy and losing all the TP history it had built up for me.
I wish there was a way around this when you are selecting several textures on a cluttered screen and you're really focused on your build. You have to have the edit window open not minimized sometimes, and the inventory open to drag textures from it. Then with a HUD like Prim Docker or Mystitool there's hardly any room left for selecting things in the world.
HUDS really should be no modify for this reason.