Apple has debuted a new support program called (quite reasonably) the MacBook Bottom Case Replacement program. As 9to5Mac notes, there's been an issue with the rubber bottom part on some recent MacBook models where some glue problems may separate the rubber pad from the actual MacBook case.
If you have had such an issue with a MacBook sold in the past few years, Apple will be happy to replace it at any Apple Store. And if you don't want to bother with coming in, you can even order a replacement kit online, then just put the new case on yourself.
It's not a huge deal, of course, and Apple says that if you're not having the problem, you obviously don't need to replace your case, but it's good to know that Apple's got a way to deal with even a small hardware issue like this.