On occasion I've used my drill chuck to hold an end mill for doing "light" milling, because I didn't want to take the time to swap out the chuck for an end mill holder. It worked OK. Today I was using a 90 degree router bit, mounted in my drill chuck, to relieve an angle on a piece of oak trim that I had clamped on my mill. Everything was going OK -- until it didn't. Apparently the vibration, plus the side load, loosened the chuck in its taper, and it dropped onto the piece while it was spinning. It really messed up the oak piece I was milling, but, fool's luck, it didn't mar the vise or chuck as far as I can tell. Lucky me. Hopefully wiser now....a little, anyway.