About the question that asks about sinful promises: If we made a foolish or rash vow, are we to keep it? Also, can you "accidentally" make a vow? Like, if you keep thinking "don't think about x... Don't think about x" and then think it, is it binding?
In general, a person is expected to keep his word to another person. It doesn't matter if you made a vow or not, keeping your word is a part of not lying to other people. "But let your 'Yes' be 'Yes,' and your 'No,' 'No.' For whatever is more than these is from the evil one" (Matthew 5:37). Even if what you said causes you hardship, you keep your word. "In whose eyes a vile person is despised, But he honors those who fear the LORD; he who swears to his own hurt and does not change" (Psalms 15:4).
As you noted, there can be times a person didn't realize he committed to something, or perhaps forgot that he made a commitment. Under the Old Law, this was considered an unintentional sin. "Or if a person swears thoughtlessly with his lips to do evil or to do good, in whatever matter a man may speak thoughtlessly with an oath, and it is hidden from him, and then he comes to know it, he will be guilty in one of these. So it shall be when he becomes guilty in one of these, that he shall confess that in which he has sinned" (Leviticus 5:4-5). When he realizes he had broken his word, he was to confess his sin and offer a sacrifice to God. There are no sacrifices under the New Law, but just as with any other sin, you confess you faults before God. "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (I John 1:9).
In regards to Leviticus 5:4-5, consider the following commentaries:
"It is very likely that rash promises are here intended; for if a man vow to do an act that is evil, though it would be criminal to keep such an oath or vow, yet he is guilty because he made it, and therefore must offer the trespass-offering. If he neglect to do the good he has vowed, he is guilty, and must in both cases confess his iniquity, and bring his trespass-offering." [Adam Clarke's Commentary].
"That is, if a person swear rashly and inconsiderately in the common affairs of life, that he will or will not do something, whether good or evil, &c. and if "it be hid from him," that is, if he have not rightly considered the thing before, whether it was in his power or not; whether it were lawful or not; or if through forgetfulness he omit to do what he might have done, when he knoweth it, "he shall be guilty, &c." ... From this and other passages it appears, that God knowing the frailty and corruption of our nature, whereby no man is able always to stand upright, but will sometimes fall into sin, and so be liable to death and misery, He Himself of His infinite mercy to His own people was pleased to provide a remedy for them, whereby they might be freed, in ordinary cases, from the guilt contracted" [British Family Bible].
"A soul swear; rashly, as to what he will or will not do, and when it may be wicked, or not in his power to do it. It be hid from him; if he was not aware at the time that it was wicked, or was not in his power. We should be careful never to declare what is false, or promise to do what is impracticable or wicked. Especially should we avoid all rash and profane oaths" [Family Bible Notes]
"a rash oath, without duly considering the nature and consequences of the oath, perhaps inconsiderately binding himself to do anything wrong, or neglecting to perform a vow to do something good. In all such cases a person might have transgressed one of the divine commandments unwittingly, and have been afterwards brought to a sense of his delinquency" [Jamieson-Fausset-Brown Commentary].
"If a soul swear-Rashly, without consideration either of God's law, or his own power or right, as David did, 1Sa 25:22. To do evil-To himself, to punish himself either in his body, or estate, or something else which is dear to him. Or rather to his neighbour. And it be hid from him-That is, he did not know, or not consider, that what he swore to do, was or would be impossible, or unlawful: When he discovers it to be so, either by his own consideration, or by information from others, whether it was good or evil which he swore to do." [John Wesley's Notes].
"If a soul swear, to wit, rashly, without consideration, either of God's law, or his own power or right, as David did, 1Sa 25:22. To do evil; either, 1. To himself, to wit, to punish himself, either in his body, or estate, or something else which is dear to him. Or rather, 2. To his neighbour, as 1Sa 25:22; Ac 23:12. Or to do good, to wit, to his neighbour, as Mr 6:23, when a man either may not or cannot do it, which may frequently happen. And it be hid from him, i.e. he did not know, or not consider, that what he swore to do, was or would be impossible or unlawful. When he knoweth of it; when he discovers it to be so, either by his own consideration, or by information from others. In one of these; either in the good or evil which he swore to do" [Matthew Poole's Commentary].
"This law would tend to prevent inconsiderate oaths, which are soon forgotten and broken; whereby the habit of truthfulness is liable to be seriously impaired. A person might find that what he had sworn to do was either impossible or wrong. In such cases he would be guilty only of rashness in making an inconsiderate oath; which, however, as implying a vow made to God, required expiation" [Annotated Bible Notes].
A rash, thoughtless, or unintentional vow can be:
- Vowing to do something that would be a sin to keep. A person is not to sin. Vowing to sin doesn't make the sin right, nor should it be kept. "And why not say (as we are slanderously reported and as some claim that we say), "Let us do evil that good may come"? Their condemnation is just" (Romans 3:8).
- Vowing to do something that is impossible to keep.
- Vowing to do something that you were unable to keep because of circumstances you did not foresee.
- Vowing to do something that you later forgot about.
Wesley and Poole brought up a good example: David was insulted by Nabal and David made a vow. "Now David had said, "Surely in vain I have guarded all that this man has in the wilderness, so that nothing was missed of all that belonged to him; and he has returned me evil for good. May God do so to the enemies of David, and more also, if by morning I leave as much as one male of any who belong to him" (I Samuel 25:21-22). Abigal, Nabal's wife, heard of David's vow, rode out to him, and convinced him that his vow was wrong. "And when the LORD does for my lord according to all the good that He has spoken concerning you, and appoints you ruler over Israel, this will not cause grief or a troubled heart to my lord, both by having shed blood without cause and by my lord having avenged himself. When the LORD deals well with my lord, then remember your maidservant." Then David said to Abigail, "Blessed be the LORD God of Israel, who sent you this day to meet me, and blessed be your discernment, and blessed be you, who have kept me this day from bloodshed and from avenging myself by my own hand. Nevertheless, as the LORD God of Israel lives, who has restrained me from harming you, unless you had come quickly to meet me, surely there would not have been left to Nabal until the morning light as much as one male." So David received from her hand what she had brought him and said to her, "Go up to your house in peace. See, I have listened to you and granted your request"" (I Samuel 25:30-35). David did not say he had to keep his vow, even though he now realized it was evil. He thank Abigail for preventing him from doing wrong and said that she had acted as an agent on behalf of the Lord.
Also notice that none of these examples deal with your private thoughts. No one is depending on you keeping your word to yourself. To tell yourself you are not going to think about a matter is an intention, but if you do think about it again, it is only an annoyance -- not a breaking of your word. You are expecting more from yourself than is reasonable to expect.