In most civil cases, a defendant may not immediately appeal the denial of summary judgment, as it's not an appealable order. Instead, a defendant who has been denied summary judgment must proceed to trial. If the trial court misinterpreted the law, an appeals court will reverse after trial. Until then, the case must go on.
The United States Supreme Court concluded that government officials were a privileged category of civil defendant. Thus, a government defendant who has been denied summary judgment may immediately appeal the denial of qualified immunity.
Fortunately, the privileged class must still go to trial when the court denies qualified immunity based on a question of fact rather than of law:
But just because an order denying a motion to dismisson qualified immunity grounds is generally considered a final decision, it does not mean that the right to appeal that order is unlimited. If the denial of qualified immunity turns on factual rather than legal questions, the denial is not properly subject to appellate jurisdiction under the collateral order doctrine because the decision is not “final.” Johnson v. Jones, 515 U.S. 304, 313-18(1995); Wernsing, 423 F.3d at 741.Leven v. George (CA7) (here). The distinction between law and fact is sometimes tricky, but not so here:
The magistrate judge denied defendants’ claim of qualified immunity on the grounds that genuine issues of material fact existed with regard to whether Officers George and Westerfield had probable cause to arrest Levan, and whether Officer George had probable cause to use force. Although at oral argument defendants’ counsel attempted to distinguish the factual determinations from the legal issue of qualified immunity, we find it nearly impossible to sever the two questions. Instead, we think it readily apparent that the question of qualified immunity turns on genuine issues of material fact. As Justice Breyer wrote in Johnson, when “a defendant simply wants to appeal a district court’s determination that the evidence is sufficient to permit a particular finding of fact . . . it will often prove difficult to find any such separate question . . . .” Id. at 314. That admonition rings true here.While the panel's view of its role in resolving disputes is correct, judicially-activist courts would have decided the issue differently. (More here.)