Oh here's a thing that definitely could do with some fixing.
I've seen it before but not gotten round to taking screenshots (or did but dunno where they would be on my HDDs)

If you look carefully you can see a bunch of dots down the middle of the screenshot.
Its all of the slingshot stones thrown by my army slowly rolling down the hill

Hard to show with screenshots but in motion it produces a curious shimmer.
Anyway: pretty sure these should time-out and disappear, I suspect it doesn't due to the kill keeping them moving.
Have tried to find big piles of slingshot stones on flat terrain but they seem to disappear shortly after landing/stopping, likewise arrows & spears.
The really amazing thing is it doesn't seem to cause any great lag/performance issue.