If you're looking for treasure troves of interesting Fourth Amendment issues, it seems like your two go-to crimes are drugs and child pornography. This case from the Seventh Circuit involves the latter.
An informant allowed an FBI agent to assume his identity online. The agent corresponded with defendant Michael Borostowski, who offered to provide the agent with child pornography in exchange for a webcam "session" with a child. After receiving the promised pornography, the agent got a warrant to search Borostowski's Yahoo email account, revealing, shockingly, more pornography! This led to a warrant of Borostowski's physical home and some more porn, then a conviction and a 24-year prison sentence.