These are two successive screen shots from OS X during audio playback.

Soon after starting playback, notice the starting point does not have an indicator, and the yellow track selection is intact.
At about 17s, I moved the pan control, and at about 20s, I moved the gain control. As soon as I touched the pan, the vertical line at the start time was restored. Notice the little gray areas where the yellow highlight was painted over while moving the pan and gain controls.

audacity screen shot

audacity screen shot