If an attack causes less than 10% of your HP in damage, it has no further detrimental effect on you (besides damage). No concentration checks, no death saving throws, etc. If damage stops spells in your game, damage that is lesser than 10% doesn't, and so on.
Something like massive damage in reverse, I guess (I dislike this rule in 3e, but 5e has an interesting variant in the DMG... that I find a bit unecessary).
The inspiration for my (probably unnecessary too) rule comes from an old game of Marvel Superheroes RPG (FASERIP).
If I remember correctly, I was playing as the Hulk... and, after taking massive damage from some super-villain, my PC got stabbed by a mook... and died!
I really hated that.
To be fair, I have no idea if this was because of the rules, of the game master, or just faulty memory (it was more than 25 years ago).
And, now that I think of it, it reminds me of Achilles... So, it might make sense in some contexts.
Again, this kind of grittiness has its uses, but doesn't quite fit the heroic fantasy tone of most official D&D stuff. Remember that the 15th-level fighter will probably survive a fall from any height!
In fact, if you want to make 5e less "bounded" and more "epic", i.e., the 15th-level fighter simply cannot be hurt by a single kobold, you might just ignore damage smaller than 10% altogether.
I have also considered a 50% HP rule as a complement... Say, if you WOULD take more than 100% of your HP in damage, the excess damage applies regardless of immunity.
So, in theory, the Tarrasque CAN hurt a werewolf with a bite, although it isn't easy.
AD&D had something like that IIRC. If you had enough HD, your attacks bypassed the usual "magic weapons" defense.
OTOH, some small creatures could be immune to fire, and swim in magma... So maybe this rule would only work for attacks?
I dunno. Maybe it is a case of rulings, not rules ("of course the Tarrasque killed the werewolf!"), or a matter of taste.