Backlight leakage refers to the problem some screens exhibit where in a darkly lit room, and with a dark image on the screen, you can clearly see areas particularly around the edges where the backlight shines through. This is a problem with the manufacturing stage and quality of the monitor build and can also sometimes be influenced by transportation and storage. A lot of panels will show slightly uneven backlighting, with perhaps a little light noticeable at the edges of the screen or in the corners. Where it is minor, and only visible on very specific (and unrealistic) test situations it is probably not worth worrying about. Also consider that a screen might need some "bedding in" time to settle in your office before you test this kind of thing. Panel uniformity can vary from one screen to the next and from sample to sample, it is very hard to predict. Where backlight bleed is severe or very distracting, a return to the retailer or manufacturer is suggested.