I have a lightbox - SEO - Panda related question:

An eCommerce site has a lightbox that prompts users to subscribe to the newsletter on the first visit. In Google Webmaster Tools, if I do a fetch and render request, Google always sees any URL with dark overlay and lightbox popup opened. 

Do you think this can cause Panda penalty or similar issues? Anyone has any experience with this? I couldn't find any case study on Google.
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