Lots of folks are tempted to mount shelves on the wall behind their lathes and use them to hold various lathe tools, accessories, lubricants, etc.. Ask yourself if reaching across a spinning 4-jaw chuck with the jaws backed out halfway sounds like a good idea. If you must have a shelf behind the lathe, use it to store stuff you'll NEVER use while the lathe is running (eg, spray paint cans, jars of pickles you put up, chainsaws, etc.).
Better yet, don't put a shelf there at all or put the shelf at the tailstock end so you don't have to reach across the spinning chuck and workpiece.
If you're not convinced, I have some photos of the aftermath of a human sucked into a lathe. Believe me, you don't want to see them.