Oxalic acid [1]. Could cause kidney stones especially if combined with other factors that may predispose you to having them. Simply cooking the spinach can remove most of it.

Personally, I ate an amount of spinach that made me an outlier(think the maximum you could fit on a foot long subway sandwich often daily for a couple years, and that was also combined with other poor choices like gas station big gulp sodas.

[1] https://en.wikipedia.org/wiki/Oxalic_acid