Smart dogs are working dogs. If they've got no work, they'll get up to bad things usually because you've left them in your house all day. If you want a pet, get a dumb dog. Or, a very lazy breed, like a greyhound.
I do wish I had a border collie (very intelligent working dog) in my life though. I should go round befriending local farmers so I can play with their dogs :D
The ones that are too smart outsmart the owners, which some dog owners see as a bad thing.