I just pulled an Italian specialty cake out of the oven and was disappointed to see that it has sunk badly in the middle. It's not to be frosted or layered, so it might still be quite good, but what could have caused the sinking? Here's the recipe, but I deviated considerably in the making of the puree. Orange Olive Oil Cake  EDIT: It wasn't pretty, but it was awesomely good! Maybe the best tasting cake I've ever made. My guest loved it too. It was very orangey, but not too sweet. Moist and dense, but not heavy. It was sublime.  EDIT #2: Here's a picture of a slice that shows just how badly the cake sank. I can't complain too much since the flavor and texture was so spot-on great, but I'd still like the final evolution to be pretty and tasty.   
A possibility would be that your cake ended up being too moist with liquid.  Note that @rumtscho reduced most of the water out in her attempt.  If I recall correctly (it's been years) Bakery Master, Joanne Chang said something to the effect of:       It's not that cake collapses in the middle, it's that the cake   holds on to the form better on the outside.   This the same reason that some cakes go the other way and dome up in the centre and why the bunt form has a hole in the centre.   What possibly happened to your cake was that the cake had a nice chance to 'cure' around the outside since heat was more accessible.  In the centre, however, being the most buffered part, the cake remained mushy long enough to let gases escape before the protein structure formed strong enough to hold it up. 