If your bug key is connected to an input port of an FPGA, you will observe the signals such as in the figure above. (Please click the figure to enlarge.)
You will notice that in some cases (see the green circles) we do not have contact bounce at the rising edges, but you can not always expect it to be true. Also note that (see the yellow circle) sometimes we have relatively long bouncing.
A simple logic is employed to eliminate those bouncing without introducing a delay. See the signal OUT_KEY.