I find that right clicking an area with a player corpse nullifies the click sometimes. I thought it was my mouse, and I freaked out since I just bought a new one.

yup i did to every now and then its as if i lag out but its not a connection issue it must be within the last patch or 2

This exact same thing happens to me every couple minutes. It doesn't always effect the game that much, but in some cases it has lost me the game. (ex: playing adc, in a late game teamfight i cant move, blitz grabs me and I die. gg)

I will never stop bumping this until riot fixes this issue. @Riot PLS

I've only noticed it with mouse clicking items or the + when leveling an ability. I know I should use the number keys and CTRL+Q/W/E/R but... meh.

This happens to me when trying to use targeted spells, buy items, and level up skills. I always thought I just failed at clicking, lol. I don't really have an issue when walking, as far as I know.

Its not a connection issue, because the game flows normally while it happens.
and it's not a mouse issue, because the clicking is registered, the convergent arrows animation can be seen and sometimes your character even pronounces his/her attack/move quote.

I just got stuck for like 15 seconds with fiora and she went "sharp blade, sharp mind" on me while I furiously clicked and she didn't even took 1 step.

I've had the same, but it also extends to graphics as well, where sometimes it looks like I'm dropping fps from 60 to 10 or so, except my fps indicator shows no such thing. I've been attributing it to dropped packets, although I dont know if that's true.