You have to be more active than just stumbling across it. If you stumble onto CP accidentally and either report it to the police (only) or delete it and show no one, you're fine. Read the USC if you're curious about the details, but no, you can't get busted just because you ran into it.
Entrapment only comes into play if you convince someone to commit a crime they weren't already predisposed to commit. So if you give them a link that says "here's CP" and they download half the site and add it to their collection, it's hard to argue that the cops made you collect CP. In general, if all the police did is provide you a chance to commit a crime and you did so, you were busted, not entrapped.
Entrapment only comes into play if you convince someone to commit a crime they weren't already predisposed to commit. So if you give them a link that says "here's CP" and they download half the site and add it to their collection, it's hard to argue that the cops made you collect CP. In general, if all the police did is provide you a chance to commit a crime and you did so, you were busted, not entrapped.
I suggest this as an entertaining introduction to the law on the concept: http://lawcomic.net/guide/?p=633