The bug happens sometimes when I log back into my Minecraft world. Sometimes, some of the trees have dark patches around them even when there are torches already there. The bug goes away whenever I place a torch or a block (I have to do it around the base of every tree affected) and it is very frustrating having to place down a block and break it every single time the lighting bug occurs. It was happening before the Nether Update released but I can't remember in what update it actually started appearing in! The bug doesn't happen every time I go back into my world, but it occurs around 30% of the time. As well as trees, it also occurs both on the outside and inside of things I have built.