I too have the vertical yellow line on a samsung t240, right in the center. I tapped the top of the monitor after seeing a post where someone said they were pressing on the monitor and tilting it. Sure enough the yellow line went away.
Why is there? I can only guess. Could be a bad solder connection or a bad connection on some internal cable. It could be a bad IC which functions as the controller to each pixel, but when you press on the monitor to fix it, that suggests it is a physical connection issue. It appears a whole column is not set correctly, so that will mean something to the designers.
I am going to rma this one. Samsung sent me a replacement on another monitor that failed before. This T240 is only 3.5 months old. Starting to wonder if samsung has found the limit in engineering a product for the least amount of cost!