This isn't really much a post yet, but I figured this was too interesting a thing to leave out:
If I ever have to do a blood oath, where’s the best place on the palm to drag the knife?
Multiple helpful answers on reddit.
Other interesting observation -- first search result is about getting help for self-harm. Obviously these are very different, but the different attitudes toward such a gesture are also manifest in the post's comments.
Now, how would this figure into a story about some impulsive characters...