As Chrono79 said 0.66 is an example. Moreover it's a question not an issue. Since 26753 guys passed the kata it seems it's understandable. Maybe you should read the description again.

Why is this an issue and not a question instead? 0.66 is an example, the value is passed as a parameter of your function and you need to validate it.

I'm sorry but I can't understand the question. It would seem to me that the ball will bounce several times, perhaps to 0.66 of the height of its former bounce height. This has to be the case, right?

I blame the precompiler for not being allowed to do this :D