What's the most most mysterious bug you've ever encountered that later turned out to have a trivial explanation?
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Advert web banner: two images side by side were reversed in position depending on which screen you used - even with same device plugged into the screens. Dragging the browser window from one screen to the other made the images flip around! It was so weird to watch it happen.
Turns out it was one retina screen and one not-retina screen in the setup, AND some position code had been accidentally written inside a media query bracket for only retina screens instead of outside of it.
