This has now happened twice and I cannot explain why, nor fix the problem.
I built a buffet for the kitchen (box on legs with shelves). On top of the box is another shelf supported by five legs (each corner and then one in the back). This top shelf is bowing upwards! I suppose the best way to explain is that if you were looking at the shelf, which is 15" deep and 42" wide, it is bowing up along the Y-axis (15" deep). I could imagine if it was bowing down due to gravity and not having enough support, but bowing upwards makes no sense.
I threw it away, re-built it, and it is doing it again. Below are some details of each build so maybe someone can identify the problem.
The first build:
The top shelf was a 15" x 42" board I bought from Home Depot. On top of that board, I added six large tiles that were left over from the backsplash in the kitchen. I then added some 3/8" strips of wood around the tile to edge / make sure everything was level. I used a spray paint to finish.
I built this in my detached garage in the cold, then brought the finished buffet indoors and put it in the kitchen. Within a few days, the shelf began to bow up pretty significantly so that the tiles no longer sat flat.
I thought that maybe doing all this in the cold was the problem, so I brought my tools into the basement and did the second build.
This time, rather than using a board from HD, I bought separate boards and joined them with a kreg jig.
Rather than laying the tile on top and edging with strips of wood, this time I routed out the area where the tile would sit. I still used some thin strips around the tile to hold them in place.
I painted with a roller and finished with polycrylic.
When finished, I moved the buffet up into its position in the kitchen. It's been there for a week, and while I wasn't watching closely, today I can see that again it is bowing upwards again.
So I don't think it is the temperature difference after all. The only very strong connection between the two builds is the tile I am using, but I have a hard time imagining how that would cause the shelf to bow upwards.
Here's a link with some pics and details on the first build. This is all pre-bowing.