A quick synopsis from the link above which has pictures to better describe the issue

"Hi All, setting up some characters. 

Most all expressions are symmetrical. There are options for mirroring, but not all of them work. In particular, mirroring Iris positions are critical as a slight misalignment really stands out. I position the first one then select Mirror option and both current and all expressions doesn't work for those views. They do work for looking up and down. Any thoughts?

Also, many of the mouth positions don't mirror well, but this is less critical. "