Haha, nice try Sweet Creations, but you are not going to get me in on that debate, haha!
Seriously though, it actually seems to be more of a problem due to dry air. Like in the winter with a furnace or heat on, but then sometimes with air conditioning and de-humidifiers, that can be as bad.
It is really just a theory of mine, but I know a lot of folks that use the water in the icing that have this issue, especially with wedding cakes that are usually iced a day or two longer before the event. The other situation we have been able to figure out, is with crumbcoats, when a different consistency of icing is used for the crumb coat, say there is more water added to it. Then the final coat, less water added to it. Well frequently this seems to result in cracking issues. Now not for everyone, but then, everyone has different humidity or dryness issues. And again, this isn't so much an issue in a cake that is iced close to the event, it is when the cake will sit iced for more than 24 hours.
The folks that have these issues, well they have been decorating a very long time and are using substantial bases to support their cakes, so that isn't the issue.
So my theory is, when there are different consistencies of icing, the one less moist layer extracts moisture from the more moist layer of icing. It makes sense, if you think about it, because as an iced cake ages, the cake moisture level changes with the act actually absorbing moisture from the icing, as the cake dries out.
Now funny thing, but I have noted that folks that use milk or cream in their icing, as the liquid, don't ever seem to report this problem. I know it has never happened to me with a wedding cake iced in advance.
We know that a cake sitting uncovered will eventually end up with dried icing and cake due to the moisture being evaporated into the air. A cake sitting in a cardboard box for days will also dry out, partly because boxes are made of paper and the paper tends to absorb some of the moisture and partly because the boxes are not air-tight.
Many years ago, I started sealing boxes in plastic bags or plastic sheets and I noticed that the icing itself stayed the same texture for days and the cake was always fine.
A few friends of mine that use the class icing recipe with water, were getting very crunchy or drier icing on their wedding cakes and I suggested bagging the boxes. This worked out well, because the icing stayed the same consistency even when the cakes were made 2-3 days in advance of a wedding date.
Haha, so I am not saying use milk or cream, just take extra precaution when you use water in your icing, make sure your iced cakes are well-sealed. I tend to use green unscented trash bags, haha, but you can use plastic sold by the roll, the kind you use as a drop cloth when you paint or you can use kitchen bags or even clear leaf bags. Haha, we have to dispose of leaves in paper re-cyclable bags here but every year hubby forgets and buys the clear plastic leaf bags. So I have a huge box of them and these are really nice, they are big enough to fit any sized box and don't look so much like you are storing square garbage on your counter.
I also find this a great way of freezing cakes, I tend to wrap up cooled cakes and double bag them to protect against freezer burn and food transfer odour. Also when refrigerating a boxed cake, this will also create a barrier to shield against moisture loss.
Rule of thumb when using butter and milk or cream in your icing, make certain your room temperature is below 75F, otherwise refridgerate. If the temperature is over 83F you will have melting issues with the butter. Otherwise, 2-3 days at room temperature, the cake is fine. I find even longer, 4-5, but I wouldn't do a cake that far in advance.
Hugs Squirrelly Cakes