Today I spotted a contextual advert within an email thread in Gmail. Usually these 'sponsored links' are displayed in a separate pane on the right-hand side of Gmail - i.e. outside of the actual email content. But this places the ad squarely within the content of the email thread. It's possible this has been around for a while, but if so I only just noticed it. I took a couple of screenshots - let us know in the comments whether you've seen this before.
Screenshot showing full ad - this displayed above the message I'd just sent and below the previous message in the thread.
Close-up screenshot showing a bit more detail of ad placement.
Note: When I re-entered the email thread, the ad above no longer displayed inside the thread. So I've not been able to re-create this scenario.