Stupidity is...
... not thinking about the consequences of what you do. Selfishness and laziness in the wrong places, and with a disadvantageous cost to others/benefit to yourself ratio (that's a ratio that has to be considered, too). Stupidity is... bringing practically no benefit to yourself and a lot of damage to others.
I just spent half an hour trying to understand who sent some client's KYC documents and for whom they were meant exactly (here we're almost 100 people). Nothing was specified on the envelope. So I called up the person most likely to have sent it, the portfolio manager for that client, but he hadn't sent them. It turned out that the Corporate department had sent them (because the client was also linked to a supermarket, and the supermarket was a Corporate client), but they didn't want the portfolio manager to know, since he wasn't even aware of the KYC process (and he complained to them because they didn't let him know).
Now, this is what happens when you're dealing with stupid and lazy people (being lazy and sparing yourself work in the wrong places means in fact being stupid): the sender sent us an envelope, without indicating sender nor addressee (within 100 people working at Compliance), in order to save himself another 30 seconds of work, or maybe just because he's so stupid that he didn't think of it. At that point what am I supposed to do? I will have to find out whom I have to give the envelope to, and to find out that, I need to find out the sender first, and to do that, I have to ask a few people, and, in the process, people the sender didn't want to be aware of his letter will find out about it. And this is a serious problem for Compliance matters, that need to be kept as secret as possible.
So you, the sender, spared yourself 30 seconds of work, and as a consequence, you got 10 people involved, wasting a collective 60 minutes, informing the wrong people of what you were doing... we're all surrounded by such idiots, and we should make an effort to avoid them.