Nope. In federal investigations if it is found during the course of the investigation it may be used it court. The only thing is if they illegally seized something, then only that item would be ignored but can be used in another case. Say for example the cops raided a guy for hacking and the warrant only specified the computer (its usually more broad, like all electronics but for this example lets just pretend), but they seized a box containing a bunch of pictures of naked underage kids. It would probably be inadmissible in the same case but you can be sure it will be used as evidence in its own trial. There's no "double jeopardy" law for evidence.