The fundamental problem with this question, as with most general conceptions of the middle ages foisted upon the public by entertainment media (and many historians) since the 19th century, is that there was no singular form of governance, law, culture or habit in middle ages until at least the 14th century (and even that is iffy).

None.

Zero.

Although it's distressing to think this, and seems counter-intuitive to the modern era where so much seems consistent and easy to grasp wherever we look - and thus it must have been so always - in fact, the consistency of our ideas of the middle ages has been created.

So, the idea of a hierarchy of nobility who acted in a certain fashion consistently, across hundreds of years and hundreds of thousands of square kilometers of geography is a fiction. The worst of these fictions is something we call 'feudalism'. Feudalism was invented as a way to explain political interactions by historians several hundred years after the fact, it was not a fixed 'system', nor would medieval societies have recognized these fixed 'feudal' conceptions of their 'systematic' relationships. Certainly a systematic understanding of the place and role of the 'knight' didn't exist.

'High born', 'low born', what are these terms? They are certainly not the way these members of society would have referred to themselves, according to what historians know. We have no idea what their interactions would have been like at any tournament ^1. If anyone claims otherwise, they are inventing things.

^1 what we do know comes from a few paltry sources which we must induce cultural meaning in to, so von Eschenbach and de Troyes' proto-Arthurian works about Percival and so forth, from the 12-13th century; we don't actually know much outside these texts and how they bear on broader culture. By the time the late 14th century rolls around we have Froissart's Chronicles, and while they do provide a lot of colourful detail about events such as jousts, they are often fanciful ideal-types of nobility prancing around in perfect tune to chivalric ideals; this tells us more about Froissart and his audience than it does of the historic moment.

Well let me give one close, but not exact example.

Gabriel, comte de Montgomery, seigneur de Lorges was a high born noble who killed the king of France in a jousting tournament. He was pardoned, but was socially disgraced, and left the court.