When a unit has many promotions, not uncommon for complex end game units, the icons don't fit well into the unit display in the bottom left of the screen. In particular, when you try to mouse over an ability/promotion icon to determine what it is, it can be obscured by the circular unit image and the tooltip picks this up instead.
This is problematic as different abilities tend to use the same symbols! I don't know another method to determine which identical ability is on a unit except through this tooltip. This means it is impossible to determine the promotion except by going into the civilopaedia and guessing which it is.
I've attached a screenshot, unfortunately the mouse pointer is not shown clearly. The mouse is however over the two leftmost red triangles trying to acquire a tooltip.