Its simple... get a stack of units with the same promotions, lets say combat 1. A subbox on the promotion in the bottom right of the combat 1 promotion icon has a number stating X number of units in the stack have it. If you highlight the number on the promotion (and not the promotion itself) it suddenly diplays a completely different promotion... leader by warlord, wooze steel, poison arrows... even if you already have that promotion on the unit it can pop up again, as a promotion count display.
The unit doesn't actually get said promotion, the promotion count is just displaying a different (in the cases I saw) promotion and so far they'll all been the special ones like warlord promotion, event granted specials like poisoned arrows. I think what is happening is that no one ever bothered to assign anything unquie to the box so it just dispalys them at random.
It should be displaying that X units have this promotion or nothing at all.
I'm not going to screen shot as its a waste of effort to bother... At the most basic this can be replicated by making a new game choosing an aggressive leaders and start as minor Civs... the put your stone thrower and clubman on the same square and shift select them into a stack. They'll both have combat 1 so the stack display will show the combat one promotion with a '2' on it. Highlighting the 2 will show a different promotion than combat 1, but not grant it. Highlighting the section of the promotion that isn't cover by the number will show the actual promotion.
