Agile vs. Waterfall

Yesterday we hung up a magnet board (of a German discounter) on a wall. This simple task led to the following learning experience:

Hanging something on a wall (which includes drilling a hole) should not be done the agile way but rather by the waterfall model.

As validation and QA happened after the first iteration we now will return the magnet board, but therefore we have two screws at a prominent place in the wall.