So...
I have a new fort on a stream that cuts through a hill, and my entrance is in the face of the hill that the stream cut.
I figure, I should channel away some of the stream to act as a natural defense, that's a smart idea!
I channel away most of the stream, being careful to designate 1 square wide strips so my miners don't get their feet wet, and everything is fine. I leave one strip unchanneled to allow outside access until I get the bridge built.
I also have a quantum stone dump right at the entrance in a tile that's next to the stream. I just dug out the main dining hall, and I mass designate the limestone in it to be dumped. I noticed the dwarfs doing something odd... HEY! stop throwing my limestone in to the stream! face palm. I forgot that dwarfs will throw any dumped item down a z-level if there's a place to do so next to the garbage zone where they can. No big deal! I have a ton more lime stone if I need it.
So, I get the bridge finished, and I designate that last strip of land, which is next to the bridge, to channel. One of my starting miners, and my expedition leader responds to the call, and...
Did you know, the stream surface can cave in if it's unsupported, despite the fact that channeling it away produces stone as though you had dug out solid rock? Did you know that a bridge cannot support said stream surface? Did you know, a dwarf mining out a 4 tile strip from east to west will mine the west most square, then stand on the third square from the right while mining out the remaining two squares? Guess who decided to go for a swim while unconscious? And, of course, when he died, he screwed up all my noble positions because he was the expedition leader.