Did you take it out of the pan when it was still pretty warm?
Sorry I thougt that was two cakes guess it wouldn't be because it was too hot. Anyway I see you started another post.
I've never seen that happen. Hmmm.
Okay...I'm sorry but is it inappropriate that the picture of this cake literally has me in tears laughing????
Okay...in all seriousness...I had my sides cave in like that when I used a heart shaped cake baking strip thing (no bottom you just pic your heart shape size and set it on a cookies sheet, fill with thick cake mix and bake - two thumbs down!!! - it doesn't work well at all). I thought it was because I may have opened the over door too many times. Looks like you had maybe too much mix in your pan and no heating core or upturned flower nail???
Better luck next time.... in the mean time if some dog lover wants a cake resembling a pile of....well....I'm just saying that cake still has some usefulness.....
Hope you get to the bottom of what happened.... muah!