I am in the process of baking 2 - 12". I'm baking one at a time. I collared the pans using Parchment paper. (BTW, this is my first time for collaring) I just took the first layer out of the oven and it has fallen in the center. I know it is not underbaked and I used a nail in the center. The cake was from white cake mix with pudding and sour cream. I use this all the time and have never had a problem with it falling. Did collaring the pan have anything to do with it? The 2nd cake is in the oven and it looks like it's doing the same thing. I will be able to level the cakes without too much trouble, but what a disappointment. I wanted a nice high cake and now I have to start cutting them down! I am Bummed. 
I had the same thing happen to me this week! I had a nail in the center and it still slumped. I used the magic strips on the outside. I didn't collar but they were deep pans.
I'll be watching to see if anyone has any advice!
I just got the 2nd layer out of the oven and it seems to be ok, so I guess the collaring didn't have anything to do with it. Now I'm wondering if I maybe forgot to put all of the water in the 1st cake? 
I took a very small triangle out of the center of the 1st layer and it looks and tastes fine so I'm sure in the end the cake will be ok. Thanks.
This JUST happened to me!! I am so frustrated!! I used the heating core, used the baking strips on the outside, used the 12" pan!! I am tired of baking cakes today!! I just want to decorate now!! Guess I will stop whining now and get busy!
Hi Price,
I felt your pain while reading your post. I once had 3 1/2 sheet cakes fall in one night 
I line and collar all my cakes with wax paper...much cheaper and works just as well.
Sometimes, cakes can be so irritating 
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