I covered a cake in butter cream then refrigerated before covering in fondant. However, as we all know you should never refrigerate a cake already covered in fondant. So my  question is, what can I do if the butter cream softens when the cake is not refrigerated and the fondant becomes uneven where it appears that the butter cream has slipped under the fondant ? 

AThe only two suggestions i have are for you to take the fondant off and start over, or leave it on and pray. If you used a stiff buttercream you should be fine.

I agree.  I always refrigerate my fondant covered cakes and your icing underneath shouldn't slip because it should be thinly applied.

Thanks for this ... When you say "stiff butter cream" what do you mean ??
 

I always refrigerate my cakes, every single time, all the way up until it's time for delivery (or sit it out for a bit). 

And....I put a thick layer of BC under the fondant too. I would expect my customers that get fondant would appreciate the same amount of BC as a non fondant covered cake. 

Depending on the amount of water or milk you use in your BC it will be soft (runny is an extreme example, falling out of your pastry bag) or almost no water/milk makes stiff, hard BC. Too stiff is hard to pipe. HTH

AThanks guys for all your help