Interesting!

You know, there is actually one other thing I've heard called sorrel, and it has that same vaguely arrow-shaped leaf, except more bulbous -- and tastes lemony sour. It's *small* and would pop up in the garden here and there. We just called it "garden sorrel". Having trouble identifying it though.

...and aha, I think it is in fact sheep sorrel! The photo on the Wikipedia page was entirely unfamiliar, but other images like this one are dead-on.