Based on some of the responses, I guess I'd have to say 'how on top'?
A lot of respectable shooting people teach to put the toe of the stock into "the pocket" of your shoulder. When your arm is in shooting position, and the elbow is up especially, there is a little notch right under the collarbone and just before you get to the shoulder joint. Use your other hand and you can feel it.
Anyway, depending on the angle, shooter's build, etc. sometimes this looks real funny.
For rifle fully on top of the shoulder, I have see a few people doing so when wearing riot shields and the like, to get the gun high enough because they cannot tuck their head in.
There is also something about free-recoil with certain shotgun trainers these days, but I can't recall the point of this.
Additionally, it can be seen as the first transition from "indoor ready." You carry the gun almost totally vertical, but with both hands on the gun where they normally go. The stock is up real high, so when you rotate to fire it can be on top of the shoulder. You are supposed to then transition to a normal stock weld, but...
And, some people just don't shoot right.
Post images of the stance in question, if available.