I didn't read the whole thread, so sorry if I'm repeating a point already made, but...
Once I had a knife with a blade that swung open very easily. I carried it tip up. The blade opened about 30 degrees without me knowing it while standing. My pants got tighter when I went to go sit down, and it nearly jabbed the blade into my thigh. Luckily, my reaction to stand back up was quick enough for it not to cut me.
EDIT: Now that I think about it, this could probably happen with a knife tip up or tip down...
Once I had a knife with a blade that swung open very easily. I carried it tip up. The blade opened about 30 degrees without me knowing it while standing. My pants got tighter when I went to go sit down, and it nearly jabbed the blade into my thigh. Luckily, my reaction to stand back up was quick enough for it not to cut me.
EDIT: Now that I think about it, this could probably happen with a knife tip up or tip down...