Recently we did some user testing on our current method of displaying data. We had a list of items in a table (yeah, I know. Our software currently uses tables heavily), with icons on one side that people could use to turn things off or on. Above, we had text that told them what to do (something like, "Click on the icon to activate!"). We tested 3 users, and all 3 thought the icons were status indicators rather than something they had to activate.
I asked the tester whether or not they used the mouse to hover over the icons, thinking that maybe if we made them more obvious with some sort of hover state that they would click on them, but they did not. Apparently, they just looked at the table and didn't try to do anything at all, they just looked at the icons and saw things as being 'on' or 'off'.