That's a good example. Shockingly poor foresight.

It's similar to the "A gun is always loaded" principle. Unless you can tell at a glance that a firearm is not in a ready-to-fire state, you treat it as if it is. People are clumsy and forgetful, and you only have to mess up once.

Leaving the live rounds right next to the blanks, in that way, is pretty indefensible. (Ignoring of course that blanks can still kill, for this analogy.)