How to replicate:
1. Use a retina Mac
2. Use shift-control-command-4 and draw a box around an area of the screen to take a screenshot
3. Paste the screenshot into Phab
4. View full-size
5. Compare what Phab displays to the thing you took a screenshot of.
Example:
{F382982, size=full}
This is annoying when using Phab to share mockups or talk about pixel-perfect UI stuff.