I once made a few cakes for dessert — some coffee cakes. The recipe that I tried was not accurate; it said butter the pan, but should have said butter then flour the pan. Half the cake came out and half of it didn’t, and it had a big crack on the side.
So if a cake flops, what can you do to save it? If the cake is supposed to be frosted, then don’t worry about it. Just cover it with frosting. It will still be delicious. If it’s like a coffee cake, which doesn’t get frosted, preslice and serve it plated with a scoop of ice cream or whipped cream and berries.
Always remember this rule of thumb: If the cake’s not gorgeous enough to be presented whole, just plate it. Be flexible enough to change the end result and don’t get stressed out. There’s no reason to tell your guests! This is what I do for a living and it even happens to me at home. Professionally, it’s different, of course — I’d have to make the cake again.