It is always a temptation to take short cuts. You may choose to paint over wallpaper either because you don't want to take the time to remove it, or perhaps the paper hides cracks and defects in the walls of your older home. Most professionals will suggest that you remove the paper because it may peel or the wall covering texture will be difficult to paint over.
Bent nails often result from poor hammering technique. However, they can also be caused by a dirty hammer face, especially when using cement-coated nails or working around adhesives. If you have problems, occasionally run a piece of fine sandpaper or emery cloth over the face. If you keep the face clean you will gain more solid contact with the nail and will avoid black marks on the wood caused by a dirty hammer.