As a woodworker I am very aware of the iron-clad rule that "wood moves". So I am embarassed to admit that I did not consider it when I built my TBHs. I built Tanzanian (straight-sided) hives and for my own reasons I joined the ends so that they sit about 3/4" above the sides. I carefully measured the width of 30 top bars and made the sides exactly that long, but I didn't consider that wood moves. In fact, wood moves back and forth laterally as it absorbs/releases moisture, but is pretty stable lengthwise. Since the top bars are laid in place side by side, they move considerably along the length of the hive. When I put the first hive out at the farm, on my first return trip the top bars were wedged tight into place and it was devilishly hard to get one out. I consider myself lucky that it didn't rain and the top bars didn't build up enough pressure to break the hives apart (which has happened to a lot of well-made but poorly-designed furniture). When I put the bars back, I found that I could leave one top bar out entirely and the rest fit quite nicely. The next week, it was tight again. That time I left the last top bar sitting in place on its side to fill the gap, and it has been fairly stable since then. So the top bars expanded the width of about 1.5 top bars over a hive with 30 top bars, or 5%. Yes, wood moves.
I still have both ends higher than the sides, and it is still a problem because it's difficult to keep the end sealed up tight when a top bar is too wide or too narrow to fit the last gap. I toyed with the idea of putting a strip of sheet metal across the ends of my hives so the top bars could slide over it when they want to expand. I will probably do that next year, but for now I have a follower board (actually just cardboard) in place so the bees don't have to defend such a large hive.
Any hives I build from now on, I will be sure to make the back end at the same height as the sides so that the bars can slide over them when they want to.