The Mechanical Appliance Company v. Castleman case in 1909 revolved around a patent dispute. The Mechanical Appliance Company claimed that Castleman had infringed on their patented invention, which was an improvement to the design of railway switches and signals. However, Castleman argued that the company's patent was invalid because it lacked novelty as similar designs were already known prior to its issuance. The Supreme Court ruled in favor of Castleman, stating that for a patent to be valid, it must demonstrate novelty and non-obviousness over what is already known or used by others in the field at the time of application. In this case, they found no significant difference between the patented device and previous devices; hence there was no new invention or discovery as required under U.S law for granting patents.

In the dissenting opinion for Mechanical Appliance Company v. Castleman, it was argued that the majority's decision to uphold a lower court ruling against Mechanical Appliance Company was incorrect. The dissenting justices believed that there were significant errors in the trial process which had unfairly prejudiced the jury against Mechanical Appliance Company. They contended that these procedural mistakes should have been grounds for overturning the verdict and ordering a new trial. Furthermore, they disagreed with how patent law had been applied in this case, arguing that Mechanical Appliance’s patents were valid and infringed upon by Castleman. Therefore, they felt that justice would be better served if their interpretation of patent law was upheld rather than dismissed as invalid or irrelevant.