What are the most common causes for encountering an 'invalid site key' error with reCAPTCHA?

The most common causes include: 1. **Typo/Incorrect Key:** The site key entered into your website's code has a typo or is not the correct key associated with your reCAPTCHA registration. 2. **Domain Mismatch:** The reCAPTCHA key was registered for a different domain (e.g., example.com) but is being used on another (e.g., staging.example.com or www.example.com vs example.com). 3. **Using Secret Key as Site Key:** Mistakenly using the reCAPTCHA secret key in place of the site key in your frontend code. 4. **Key Deletion/Expiration:** The site key might have been accidentally deleted or expired (though keys typically don't expire). 5. **Environment Differences:** Using a production key on a development environment, or vice-versa, without proper domain registration for both.