I have seen all too many times wheel spacers come off during autocross, road course racing, and even on the street. I don't have videos of these events as they all seem to happen at random, but here are some videos to illustrate my point.

https://www.youtube.com/watch?v=Vq2H-Ox_F_I

https://www.youtube.com/watch?v=jv4m41viy4I

oh and also to illustrate the multitude of manufacturing defects that are usually apparent in these:

https://www.youtube.com/watch?v=pCrog2riJ9o

Not to mention they aren't legal in some states here in the US....