Introduction of such evidence would be consistent with the pleadings under the Federal Rules, even if A does not file a reply to B's answer. A, a citizen of Iowa, sues B, also a citizen of Iowa, in an Iowa state court. A asserts one claim that is based on federal law, and another totally unrelated claim that is based on state law. The federal court may determine the federal claim, but it may not determine the state law claim. A and B, as plaintiffs, then sue C in a Minnesota state court.
C removes the case to federal court.