I recently shot 8x10 in a stand of spruce trees. The trees were beautiful -- incredibly parallel and vertical. So of course I slapped the level on the rear standard to get the camera nice and square. I got the talent lined up, took a few sheets, recomposed, re-leveled, and took a couple more.

Looking at the negatives, the trees are leaning to the left. The trees themselves were leaning slightly (1-3°) to the south. The shot is technically leveled, but the subject matter is not.

Meanwhile in the studio, I've used the level to get the camera nice and lined up, only to get back negatives with portrait subjects leaning to one side. The problem? My studio floors lean all kinds of crazy directions, and so do my subjects.

The lesson? I'm throwing away the &$%* bubble level! From now on I'm only trusting the lines on the GG, instead of leveling my shots in a non-level world. If it looks straight, it is straight.