Serious barbecuers use different wood species for smoking, to impart the desired flavors. A problem can arise if they forget what species they last loaded into their smoker. That's why barbecue supply company The Pit Bros sell magnets like these that you can slap onto the side of the hopper:
It's effective, if inelegant. A huge design improvement is this 3D-printed Pellet Pal:
Takes up less space, uses less materials, and cheaper than buying the individual magnets. Win-win-win.