This is an addition to "you are teaching when you don't know it". I got a blog from a newbie the other day and the handler was teaching a pup short retrieves and delivery in the house. I think it was also to demonstrate how to use the collar for recall. The dog came right back, put its front paws in a small stool (I don't know why) and presented the dummy. On one of the exchanges the handler dropped the dummy, and not thinking, she picked it up and went on with the lesson. NEVER do this, the dog ALWAYS picks up anything that is on the ground.
When I'm teaching sight blinds, I always fumble the dummy and the dog grabs it almost as soon as it hits the ground (she thinks that it is great fun) and hands it to me again.
I'll let some of the others tell you what can happen in a test if the dog doesn't pick up a bird at your feet. You are always teaching.