I've literally been in one of those 'hey, look! baby ducks!' situation (there is a television in the office and they really showed a clip of really cute baby ducks) while in the midst of giving instructions to a junior.

Another time, was talking to a friend halfway through a sentence, got bored and changed the topic. He was like, 'Did you just interrupted yourself?', and the worst thing is, I don't even remember what I was talking about previously. :s

At work, I often leave sentences unfinished (documents) - I don't even know how that is possible!!! I'd just be typing something and then remembered I need to insert something else and I'd just do that ... it is terrible.