Happened upon a bug after one of Johnny's side missions, one where you receive his shirt, glasses, and jacket. They all have a '!' Quest marker next to them and can't be sold, dropped, or disassembled. You can upgrade them into epic and legendary items but they all stay in the backpack and can't be removed either. You can even keep trying to get better legendary versions with more slots for mods but you will be left with the ones in your inventory that are left over.