The Pharisees were guilty of obeying the letter of the Law at the exclusion of the spirit of the Law. They majored on minors and paid heed to minor points such as tithing in small matters and ignored the weightier matters of the Law: faithfulness, mercy, and love. (Matt. 23:23). For instance, one may say that he frowns upon dancing but overlooks covetousness as a minor affair or offense. Being a legalist leads to judging and even condemning one's brother or sister. We are not to compare us to other persons or our Christian brethren (2 Cor. 10:12). Legalism is one way to end up doing that!
Sometimes necessity knows no law like when in the parable of the Good Samarian and the priest didn't want to become ceremonially unclean by defiling himself or even missing his Sabbath. This is an example of "going beyond that which is written!" (1 Cor. 4:4). Besides going against the spirit of the law, that it is fulfilled in loving our neighbor, making up your own rules and adding sins is also legalism. Tradition is often added to the Word of God and counted as a way to gauge sin. The priest was a slave to his tradition and could not see a need to be fulfilled. Legalism elevates tradition to the level of Law and bind people where the Bible has left them free!
In my day, good Christians watched the hemlines, movie lines, and hairlines! Spiritualty was something to be seen by men and an external thing noticed by others. Basically, believing that faith plus works equals salvation is legalism; actually faith equals salvation plus works--viva la difference! One must distinguish that legalism sees sins, not sin! Remember, the letter kills but the Spirit gives life! Legalism is a form of spiritual tyranny as one believes he is spiritual but actually still a whitewashed tomb or refined sinner.
We are forgiven for our sins and what we did, but delivered from what we are, sinners. The problem of mankind is slavery to sin and that can be any sin, even pet ones or presumptuous ones. When one says, for instance, to give up this or that to become a Christian, one fails to realize we are slaves to sin in toto and are sinners by nature.
The opposite and equally evil distortion of the Law of God is antinomianism! The theme song: "Freed from the Law, O blessed condition, now I can sin all I want and still have remission." Some believers may think salvation is a license to sin or that God overlooks our sins. But Martin Luther wrote a book condemning these heretics: Against the Antinomians. The only way to maintain balance between these two extremes of Bible application is to know the Bible and to understand the definition of sin and what it is not as well. Soli Deo Gloria!