What is the legal test for qualified immunity for a police officer?

"shield of immunity"²⁴ lost only in "rare"²⁵ circumstances, even for mistakenly issued warrants. It is the "magistrate's responsibility to determine whether the officer's allegations establish probable cause"²⁶that is, a "fair probability" that evidence of a crime will be found.²⁷ Qualified immunity for police officers does not even require that much, because as Ashcroft v. al-Kidd held, the shield "protects all but the plainly incompetent or those who knowingly violate the law."²⁸