Upside-Down Lemon Squares?

Question

I made a batch of lemon squares last night with my tried and true recipe that I have made multiple times. When it was done, I let it sit on the counter until completely cool and put it in the fridge.

After about 4 hours, I took it out to cut it into squares and was shocked to find it had reversed itself! The recipe has a shortbread crust on the BOTTOM and a lemon curd on TOP but when I was getting ready to cut it up, the shortbread crust was on top!

When I put it in the fridge to cool off, it was the right way, so it did this magic trick in the fridge!

There were a few changes I made to the recipe this time:

• I poured the lemon curd over the cooled crust instead of a hot crust.
• I added lemon zest to the crust for extra flavour.
• I increased the filling by 50%.

Would any of these seemingly minor changes cause this magic trick? But more important, how is it physically possible? I could see if it was different layers of a fluid cake batter that could reverse itself, but the shortbread crust is baked first so it's a solid. Not to mention it reversed in the fridge, not while baking!

It was baked in a 9 inch square pan lined with parchment paper. Does anybody want to tackle this one, so I can sleep tonight?

Answer 1

I assume the crust floated and the curd displaced around it, rather than anything actually "flipping." Actual flipping would fall under the "other members of the household" comment.

Floating merely requires the shortbread crust to detach from the bottom of the pan (perhaps aided by being cool) and lemon curd to flow under it, slowly - it may have been doing this (but not broken the surface) before it was refrigerated, and continued in the refrigerator. Nothing needs to be overly fluid for this to happen - flow in thick/viscous liquids just needs plenty of time.