In the 1991 case Hunter v. Bryant, the U.S. Supreme Court addressed issues related to qualified immunity for federal officials and probable cause in arrests. The case arose when two Secret Service agents arrested Billy Dale Hunter on suspicion of making threats against President Reagan, based on a tip from an informant who later turned out to be unreliable. After no charges were filed against him, Hunter sued the agents for violating his Fourth Amendment rights by arresting him without probable cause. The court ruled unanimously in favor of the Secret Service agents, stating that they had acted reasonably given their position and information at hand during that time even if it was later revealed as false or inaccurate; hence they were entitled to qualified immunity - a legal protection provided to government officials performing discretionary functions unless their actions violate clearly established statutory or constitutional rights which a reasonable person would have known about. This ruling clarified that officers can make an arrest based on probable cause (reasonable grounds) even if this belief is mistaken, as long as it's objectively reasonable.

In the dissenting opinion for Hunter v. Bryant, it was argued that the majority's decision to grant immunity to FBI agents who arrested a suspect without probable cause contradicted previous rulings and undermined Fourth Amendment protections against unreasonable searches and seizures. The dissenting justices believed that qualified immunity should not be extended in cases where law enforcement officials violate clear constitutional rights, such as arresting someone without sufficient evidence of wrongdoing. They contended that this ruling could potentially encourage reckless behavior by law enforcement officers, knowing they would face no personal liability for their actions even if they were found to have violated an individual’s constitutional rights. Furthermore, the dissenters expressed concern about how this decision might affect public trust in law enforcement agencies.