1. insert numbered formula with <ctrl><alt>-n 2. type some symbol, e.g. "5" and mark it with <shift>-cursor from left to right (!) such that after the marking the cursor is at the end of the math-environment. 3. pressing <del> will not delete the marked text, but remove the equation number.
This does not happen, if the text is marked from right to left. The same behaviour is in numbered equation arrays. - Sebastian
