I think I have a pretty good one....This happened when I was a new grad, working on a med/surg unit. I had just finished everything I needed to do in my (totally alert, oriented, and awake) patient's room, when I noticed that her foley cath bag was full. So, I emptied the foley, and moved along....
A few minutes later, that patient rang her call light. "Something's making a weird noise in here", she said. When I entered the room, she asked if I heard "that dripping noise". Uh-oh!

After I'd emptied her foley, I had forgotten to clasp the tubing, and there was urine dripping onto the floor! I felt awful (not to mention completely idiotic!). That was over five years ago, but it still always sticks out in my memory as one of my most embarrassing nursing moments.