Created attachment 287230 [details]
HTML and CSS to reproduce the issue
This came up while I was looking into another bug involving backface visibility. If a containing element has a 3D transform and a reflection, then backface-visibility: hidden produces the reverse effect for contained elements (that is, elements that should be hidden are visible and elements that are visible should be hidden).
The attached HTML file provides an illustration of this. box1 and box3 should be visible (and are if the reflection is removed), while box2 should be hidden (and is if the reflection is removed).
Created attachment 287231 [details]
HTML and CSS to reproduce the issue (fixed typo)