DanApp
When you are using a screen saver that includes ad links (like the scrolling Cityscape, etc ones) if you press the OK or PLAY/PAUSE button ( ►|| ) this will activate any ad that is showing at the time. If the ad is for a channel app you have installed, it will load that channel, if the channel it is not installed it will offer to install it.
It's best to get in the habit of exiting the screen saver by pressing some other key (I usually use the ✱ OPTIONS key). Or use a screen saver that doesn't include ad links.