Posters! They are the bane of existence! There is so much information you have to put on and it all has to be composited correctly for it to look good. Also, you're dealing with file sizes of 100+ mbs so it takes longer to do basic actions!
I love doing web banners and buttons because there is a lot of room for error. When you save-for-web and reduce the size, everything looks clean! Posters are the complete opposite.