A strange fire refers to doing what God has not commanded one to do or doing it in the wrong way (Leviticus 10:1). There are instances of those that offered strange fire unto the Lord in the Bible apart from Nadab and Abihu. The sons of Eli, Hophni and Phinehas, gave no honour to the offering of the Lord and His temple (1Samuel 2:12-17,22). King Saul rendered an unauthorised service to God and failed to obey God completely (1Samuel 13:8-14; 15:18-22). Ananias and Sapphira lied while presenting their seed offering before the Lord (Acts 5:2).
Eye-service, partial payments of tithe and offering, serving God for wrong motives, toiling with sin as a believer, etc are all examples of strange fire.