I'm using ConvertKit for newsletter signups, and choices are either:
1. Validate emails with recaptcha (which is why you see that thing in the corner.)
2. no email validation at all, so random people get spammed with confirma-you-want-to-subscribe emails.
I am not super-happy with having to make this choice :(
In addition, I just finished reading the Rust book last night, and I don't believe this is what interior mutability is (at least as used in Rust).