It really depends where the crawler is finding these links. I don't think this is a mistake. It's possible that people are adding these links to their site instead of "mailto:" links and crawler is simply indexing them.
It's not the only one overall, but this one likely ranks the highest. It also is bundled with gmail because it links to gmail domain (mail.google.com).
However, I can't understand how this one specific URL ended up being shown as the second link when you search for "gmail".