We have 2 sites displaying this error when trying to add an item to the cart: Has anyone seen this before?

Yes, we saw this on our current Elbrus site during development. The cause for us was if you tried clicking add to cart before the full item on the page had properly loaded.

I saw this on Elbrus when trying to add promocodes to a quote. I think it is a known limitation that you cant have auto promos in quotes