There are so many.. but probably my worst was when I was building this project and optimizing it for mobile but for some reason when in mobile view, there was a slight white line on the right side when swiping right or zooming out (shouldnt be able to zoom out either).. checked all the media queries and viewport code and everything was normal so it drove me crazy for a few hours. Turns out, I had 1 little element slightly wider than the viewport but you couldnt tell by looking at it xD fixed the css, but still so frustrating.