The Gemara in Succah comments that the Aron was a טפח thick. The Torah states that the Aron was made of pure gold. However, the Ralbag on Parshas Teruma points out that this can't be. If you add up all the gold mentioned in פקודי it is not enough gold to make the Aron an טפח thick. The Ralbag therefore says that Chazal didn't mean literally a טפח but rather meant that it had some thickness.
Here is what the Ralbag says (my translation):
It is impossible that the thickness of the gold Aronos was a טפח, as Hazal said, because it would be impossible to complete even one of these Aronos with this thickness with all the gold mentioned in Pekudei that was used for the work, and this will be clear to anyone who has studied a little geometry. But, Hazal said this to teach that these Aronos had a perceptible thickness