I have made buttercreme A LOT of times and I have NEVER seen this happen...Cake was crumb coated, then frosted as usual...
I'd say a good two hours after it was frosted (and decorated) it developed a "bubble" about the size of a 50 cent piece right on top.
I managed to poke it with a pin, create a small hole and pat the air out of it.
About 1/2 hour later, it happened again!
This time I left the hole open for quite awhile before smoothing it back out and it did not come back.
Question...why did I get an air bubble?
Perhaps it had something to do with the cake settling I imagined at first since it was filled in between layers, however, I filled & stacked the cake the night before.
Musta been somethin in the air- cause I had the exact same thing happen to me this weekend!!! I saw it and thought- "What the heck? How did THAT happen? " I dont do ANY all buttercream cakes, but this was a small little 4" round cake for a baby's first birthday, and the mom didnt want to use fondant.
Maybe someone has an answer.....
Cake farts!!!! I always get those! It's sort of a mystery here on CC. Search bake bulges.... lots of posts on the matter. Good luck