Water Damage

• Even with a finish, butcher block is vulnerable to water and can develop water spots. This isn't a problem if the water is cleaned up immediately, but when using a slow cooker the point is often to be able to leave it to cook without worrying. If your slow cooker produces a lot of condensation, steam or other drips, and that water is left sitting, it will damage the butcher block. A lot of water left too long, can also case warping and cracking.
  
  

Heat Damage

• While butcher block is vulnerable to heat damage, this is usually from contact with a pot or pan taken off a hot stove or right out of the oven. However, some slow cookers can get quite hot. If you're worried about your slow cooker damaging your butcher block, a heat-resistant pad is probably a good idea. Such pads come in many shapes and sizes, and will shield your butcher block from pots and pans as well as slow cookers.
  
  

Scratching Damage

• Some slow cookers have narrow metal feet and while many come with attached pads to prevent scratching, these pads can come off in time. Butcher block, like all wood, is vulnerable to scratches, but they can also be buffed out of most butcher block with a little effort. The best cure, however, is prevention. If you're worried about scratches, a heat-resistant pad will prevent them as well, as will coasters placed under the legs. Make sure your slow cooker sit level and isn't tipping to any one side. That can cause spillage, which can also damage your butcher block.
  
  

Butcher Block Finishing

• Butcher block is usually finished with natural oil, which does protect it against water to some degree, although it's still recommended that spills be wiped up immediately. An oil finish also has to be replied periodically by rubbing white mineral oil into the surface. Water spots and scratches can be repaired with a sanding and re-oiling.