Problem with an advert

[NobleGas]

One of the adverts that appears on the site is directing me to another website and then popping up a prizewinner notification message. I'm not sure which ad is doing it, but I end up on a page displaying this:
Fakeprizenotification.jpg

It only ever happens on this forum, so it's not adware on my PC that's doing it.

[Admin]

According to the adsense program none of the ads should be throwing you to third party sites without user intervention(a click or similar) but weirder things have happened - can you find the specific offending ad so I can report and blacklist it? This would help as I can't see the site mentioned in your image anywhere when I search the control panel for it.

[NobleGas]

Unfortunately I don't know which ad it was as I wasn't viewing the forum at the time I got redirected; I'll try to keep an eye on the ads in case it happens again.