On-Hit applying spells
The most notable abilities that come to mind are Ezreal's Mystic shot and Gangplank's Parley
It's not exactly a bug... unless it is? but more of a consistency issue that comes to light in the recent bugfix of
procing on Ezreal Q because of a magic damage issue. The point made is that it is not supposed to proc "on-spell" effects, even if it is converted to magic damage
the problem is spell shields will block it for 100% of the damage (Even if the damage is still physical) despite the fact that it is not supposed to proc spell effects - that seems to lack consistency with the idea of not procing things like
(i don't want that back, that is a whole can of worms that no one wants to balance around)