First bug: A dwarf will normally not channel out a floor tile he's standing on. That is a good thing and usually works. I have however found out the hard way that a dwarf standing on a hanging up/down staircase, one that is only supported from above with open space to the sides and below, will channel out the stairs he's standing on. Thus if you have a hanging staircase dangling above a large open space and try to remove the bottom-most stair tile, a miner will remove it and then fall to his death. I just had this happen, and other than losing a miner I'm not sure how to safely remove the rest of that stair now. It appears that the check that prevents a dwarf from channeling out the spot he's standing on doesn't apply if that spot is an up/down stair.
Second bug: A miner will refuse to channel out a space under his own feet even if he could do so safely by changing his position. I had a miner in an enclosed area (where he was unable to reach any other job site, or any meeting area or other default location) removing some hanging floor. I had some flooring I really wanted removed now, so I moved a miner in there and locked him in to prevent him from taking any other job or taking off to eat or drink. I discovered that if I ordered him to channel out the floor on the spot he was standing his status would remain 'No Job' even though he would have been perfectly safe had he moved one space over and then channeled out the floor. It appears that a dwarf will refuse to take a job to channel a floor where any creature is standing, even if he is that creature and would have to move off the spot to channel it.