can ignorance excuse a sinful act?

jesus said 'forgive them father, they know not what they do'

but if you look around on the internet, a lot of people say that a person acting ignorantly can still commit a sin. 

i always looked at sinning as 'intentionally doing what you know is wrong'. this involves intention and knowledge, something that resides in one's mind, as an opposite of ignorance. 

acts 17:22-31

22 Then Paul stood in the midst of Mars' hill, and said, Ye men of Athens, I perceive that in all things ye are too superstitious. 23 For as I passed by, and beheld your devotions, I found an altar with this inscription, TO THE UNKNOWN GOD. Whom therefore ye ignorantly worship, him declare I unto you. 24 God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands; 25 Neither is worshipped with men's hands, as though he needed any thing, seeing he giveth to all life, and breath, and all things; 26 And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation; 27 That they should seek the Lord, if haply they might feel after him, and find him, though he be not far from every one of us: 28 For in him we live, and move, and have our being; as certain also of your own poets have said, For we are also his offspring. 29 Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man's device. 30 And the times of this ignorance God winked at; but now commandeth all men every where to repent: 31 Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead.

there are sins of ignorance, and presumpteous sin.

psalms 19:12-13

12 Who can understand his errors? cleanse thou me from secret faults.
13 Keep back thy servant also from presumptuous sins; let them not have dominion over me: then shall I be upright, and I shall be innocent from the great transgression.

basically you repent over the sins you do know and have committed, then search scriptures and ask God to reveal sins you arent aware of that you commit. This is holiness and pleases God.

if you want an answer to your question however. Assuming it is true ignorance and not willful. scripture has this to say.

romans 2:12-16

12 For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law; 13 (For not the hearers of the law are just before God, but the doers of the law shall be justified. 14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: 15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;) 16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.

you are accountable to judgement by the good in your conscience and knowledge of the law and word of God.

so if a person sins ignorantly and has no knowledge of God, their conscience will trouble them, and they will know that they have done evil. If however they do a wrong and their conscience does not trouble them, yet they have broken the law of the land or of God, and they knew it, they must be held accountable of that transgression

Ignorance is not a justification for anything. Sometimes we forgive people because of their ignorance, but if they are ignorant beyond repair there is no point in forgiving them for their sins. They will keep sinning.

Yes, if you are committing sin in ignorance, you will still be judged for it.

its possible that one group of gentiles paul referred to is the maltian in acts 28

i guess there is the point, that jesus said 'forgive them they know not what they do'.... i was focusing on the know not what they do part, but the fact that Jesus asked for them to be forgiven, probably means that they committed a sin, even if they are ignorant. i never looked at it that way. 

acts 3:13-20

acts 2:22-24,37-39

22 Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know: 23 Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain: 24 Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.

37 Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do? 38 Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. 39 For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.

It's worth noting, that Jesus is not saying that he has forgiven them or that the Father has forgiven them.  We often assume that to be the case. But the words Jesus spoke, are a request, not a declaration.  It may well be that Jesus is suggesting that ignorance might be an explanation for what they are doing. 

It's worth noting in the OT that the Jews would offer sacrifices for unintentional sins.   

Like you, I would take the view that intention and knowledge are necessary components of sin. Otherwise, the notion of free will and free choice becomes a little redundant.  We often say that sin is any act or omission of the law of God.  That includes both direct acts and omissions of things as well.   Hence, hitting someone is sinful, but also not giving someone a glass of water who needs it is sinful too.  disobeying your parents is a sin - and so is ignoring your parents or not respecting them is a sin.  

Having said the above, ignorance is not an excuse in normal life. It can be an explanation. And may well provide mitigating factors.