Movie prop weapons

Alec Baldwin shot a woman dead on the set of his new movie with a prop weapon.

Brendon Lee also died getting shot by a prop weapon.

How the fack do they keep making these mistakes, how can you mess this up so that real bullets/weapons get mixed with propped.

Baldwin has be devastated by this. Could scar someone for life. Hope he is going to be ok. Thoughts are with the loved ones of this woman.