This is the assembly for a breast pump that attaches to the breast:
There is the top bit, which has the pump-y stuff that attaches to tubes on the pump itself, and there is the bottom bit, which is a bottle that has been screwed on.
On Tuesday, I set up for my third pumping session of the day. After about ten seconds, I started to wonder why the sound was different. Then I noticed that my skirt was damp. It took me seriously thirty more seconds to realize that I had not screwed the bottle onto either of the pump assemblies, which means that I was pumping milk directly into my lap.