Butters and soft cheeses do not just curdle with the addition of an acid, when you add any liquid into a butter or soft cheese and if it is added to quickly (as you are essentially forming an emulsion of liquid and fat) it will split and curdle. This is why in creamed butter cakes you will often see the instruction to 'slowly add the beaten egg as not to curdle the butter' or something similar to that.
In a cake it won't make to much difference, it will just be a bit denser, but in something that's not cooked like a buttercream frosting it will leave an unpleasant, watery, almost gelatinous mouth feel and will separate out. You can salvage it by putting in the fridge (which solidifies the fat and pulls the mixture together), then add some more icing sugar (which absorbs the remaining liquid) and give it a good mix and it should come back together.
Hope this helps!