When irregularities in an officer’s election or appointment are subsequently discovered (e.g., an officer fails to file an official oath or an officer continues to serve even though the officer’s term has expired and the officer is not entitled to hold over until a successor is appointed and qualifies), the officer is usually considered a de facto officer. Accordingly the courts have described a de facto officer as: "one who is in possession of an office and discharging its duties under color of authority. By color of authority is meant authority derived from an election or appointment, however irregular or informal, so that the incumbent be not a mere volunteer." Schoonover v. City of Viroqua, 245 Wis. 239, 244, 14 N.W.2d 9 (1944) (citations omitted). See also State ex rel. Reynolds v. Smith, 22 Wis. 2d 516, 522, 126 N.W.2d 215 (1964); Joyce v. Town of Tainter, 2000 WI App. 15, 232 Wis. 2d 349, 606 N.W.2d 284 (1999).