Example #1: The Our Father. "Forgive us our trespasses as we forgive those who trespass against us." The Lord Jesus makes clear here that our forgiveness is contingent upon whether we forgive others. But sola fide cannot properly explain this. If we are saved by faith alone, then our forgiveness is not dependent upon whether we forgive anyone anything. But this directly contradicts the prayer that Jesus taught us.
Example #2: The unmerciful servant (Mt. 18:21-35). "So also my heavenly Father will do to you, if you do not each forgive your brothers from your hearts." Note that the parable is told in response to a question asked by St. Peter: "Lord, how often shall my brother sin against me, and I forgive him?" Jesus tells the parable to the apostles. They and we must forgive others if we hope to be forgiven by God for our sins. Once again, sola fide cannot make sense of this. Once again we see that the Lord Jesus makes salvation not contingent upon faith alone, but also upon what we do. Our deeds matter, and we can lose our salvation by our sins.
Example #3: The sheep and the goats (Mt. 25:31-46). The difference between the sheep and the goats was not to be found in whether the one had faith and the other didn't, but rather in their deeds.
Now it seems to me that there is a related error - one that is not necessarily held by all Protestants. That is the so-called "perseverance of the saints," by which they mean (roughly speaking) that God's elect cannot and will not lose their salvation. So the Protestant might object to the examples given above that those who stumble on the issues raised in them are not actually the elect, whose salvation is certain.
One issue with that is that example #2 is directed explictly to Peter, and implicitly to all the apostles. There is no hint in the text of the parable that Jesus is speaking per impossible when he tells Peter that unless he forgives his brother, he cannot be forgiven himself.
Another issue with this is to be found in Hebrews 6:
For it is impossible for those who were once enlightened, who have both tasted the heavenly gift and become partakers of the Holy Spirit, who have moreover tasted the good word of God and the powers of the world to come, and then have fallen away, to be renewed again to repentance; since they crucify again for themselves the Son of God and make him a mockery."Who but "the elect" are actually enlightened? And who but the elect have tasted the heavenly gift? And who but the elect have become partakers of the Holy Spirit? It's ridiculous even to suggest that such descriptions may be applied to men to whom they never really applied at all.
More could be said. But the point is that Christians may lose their salvation by what they do. So we must pray for grace to persevere, and then we must do it. Holiness isn't something that happens to us. It is something that we must pursue. It is something that we must strive after.