openbox: disable "grab" window that moves together with mouse cursor

Normally in Openbox I can move windows around by holding Alt and grabbing the window with left mouse button. But sometimes I accidentally "grab" a window and then it is stuck to my mouse without holding any keyboard key. The window moves with my mouse, and my mouse cursor changes to a "hand" cursors. I can never find out how I did it, but it happens often enough, usually when I am doing something fast on the keyboard in the lower left corner around Alt and Ctrl and moving mouse simultaneously. Anyway, how can I disable this?