A long time ago, Barry Weeks coined a term for it -- "Progressive Sublimation" (or something like that ...). You go out to the shop to do one little chore before breakfast ... but ... the next thing you know, its 5 pm, you have crap (that you hadn't planned on doing) spread out all over the shop, and you gotten involved in a lot of work (you're tired and dirty) EXCEPT what you originally went out to do. Happens to me all the time.
Craig -- know what you mean. I have "shoot on sight" orders for the two most evil entities I can think of ... the Good Idea Fairy and the "While I'm At It" Fairy ... Both of them have caused quite a lot of mayhem in my life ...