I remember a few years back Aus vs NZ. I think it was Higginbotham going over for a try jumping over a NZ defender.. could be Read going over vs Aus, can't remember but that got blown up and I remember thinking it looked quite dangerous. The defending player was head on between the (non)scorer and the try line.
Then there was the RWC where that Samaon/Tongan(?) back went into the tackle with clear intention of kneeing the defender in the head. That went horrible! I can't remember the details though. I suppose if you went to hurdle over a guy there is every chance something similar could occur as 1) rugby players aren't hurdlers and 2) the 'hurdle' is a man in motion so..
That said I've seen many players go over tacklers coming in at pace low and from the side though they don't jump as much as lift their legs higher.. I don't know what the exact wording of the law is in this matter though so no help. Just thought I'd bring up the instances above if anyone else can remember details.