[solved] "Something went wrong" / "Not found" error when trying to add to cart

[chcsep]

I had a product that, when you try to add it to your cart, you get a Shopify error message instead of being taken to your cart:

Something went wrong

What happened?

Not Found: The page you were looking for does not exist.

The product would still be added to the cart, but the cart page wouldn't load.

After a few days' worth of troubleshooting, I tracked down the issue to the fact that the Option name (i.e. the heading for variants) was set to "Format". In my very old Brooklyn theme, this must do something on the back end that caused the cart to not load.

I'm posting this here in case anyone else encounters this issue in the future.