On April 1st, Google will begin enforcing consistency between the landing page URL and the display URL.
Google has held this policy for a long time, but it has been loosely enforced. We've found plenty of ads where the display URL is never reached when a user is redirected to the final destination. Google has been allowing some advertisers to effectively make up their display URLs.
Of course, you can tracking URLs can still be used, but the tracking URLs need to ultimately land at the final website.
Google will also inadvertently(intentionally?) penalize the advertisers successfully using this technique. The moment these advertisers change their Display URL, the ad is treated as if it were a new ad (and its positive history is lost). This typically means that the ad drops down a few spots.
We track a number of search terms where advertisers are using this technique. It will be interesting to see how quickly after April 1st they turn off these ads.