If you are using flow-children and on those children you have an element that adjusts the line-height, the size of the element appears to be calculated on font-size instead.

You end up with text overflowing the element if line-height is increased, or end up with lots of extra padding of empty space if line-height is smaller.
This is really messing up some UI stuff I'm doing since there isn't really a good hack to get around it that I can find.