Wait a minute...so, in preparation testing for the big event, the ball broke the window. So then, rather than say maybe this isn't a good idea, Elon said "Franz, just throw it a little bit gentler". So he threw it more gentle and it didn't break. Then Elon said "Franz, I've known you a long time and you have a very consistent throw. I want you to remember exactly how hard that last throw was and to repeat it tomorrow on stage so the glass doesn't break. Franz said "OK, Elon, sounds like a plan". Never mind that Elon knows how fallible human perception is.
Okaaaaaaaa-aaaa-aaaayyyyyyy.....
What actually happened is Musk has created a situation where they can show they are constantly improving their products. At a later date, they will show how they perfected it, everyone will remember the broken glass and tweet the new video around and it will be another opportunity for positive publicity. Publicity works best when people can relate to it. Without the initial glass breaking, people would just say "Humf, so the glass didn't break, big deal". But since it broke the first time, but not now, people will view it as more impressive - a big improvement. It's definitely more powerful this way. Musk understands how the human brain works and how to harness it.