I have some across a crazy IE11 Flexbox bug that I find difficult to explain. So watch this (please don’t mind the placeholder image):
Basically, when you moused over a link, the entire box would shift upwards. Mind boggling, isn’t it? This was only happening in IE11, not even IE10. In looking at the inspector tool, it didn’t seem like any margins were disappearing on the flex child, but I did have the margins on that inner container set to 11%–specifically for IE9 and below, since Flexbox centering wasn’t going to work for them. I took it off and voilà. So lesson? Don’t use percentage margins inside of Flexbox, even when it’s not the direct descendant of the flex container.