It's too common: you're approaching a jogger, get over about to pass and then abruptly without looking the jogger cuts a u-turn right in front of you. Two people have posted about it this month in the "How was your commute" thread, and I've seen it probably half a dozen times. It still scares me, and warning them doesn't always help. In the commuting thread, the jogger was so engrossed in her phone that she ignored the warning, ignored subsequent hollering, and ran right over the guy who was completely stopped and trying to get her attention. Not an exaggeration; he had video of it.
I've noted some of the tell-tail signs that a runner is about to pull this.
1) He might dip his left shoulder, or lean back with his head coming up
2) Sometimes there's sort of a left-to-right wobble as he's still jogging
3) When they slow down to a shuffling kind of walk, especially at an overpass or crossroad it's a dead giveaway
4) I think that stairing down at electronics - gps, hr monitor etc - would be a signal but I haven't actually caught this one before the rush-around so I can't definitely call it a tell.
What are some of the other warning signs?