3 There is not only this great evil in an oath, that it punishes those who are guilty of it, both when violated, and when kept; a thing we do not see take place with any other sin; but there is another equally great evil attending it. And what is that? Why that ofttimes it is utterly impossible even for those who are desirous, and even make a point of it, to keep their oath. For, in the first place, he who is continually swearing, whether willingly or unwillingly; knowingly or unknowingly; in jest or in earnest; being frequently carried away by anger and by many other things, will most surely become perjured.
And no one can gainsay this; so evident and generally allowed is the fact, that the man who swears frequently, must also be a perjurer. Secondly, I affirm, that although he were not carried away by passion, and did not become the victim of perjury unwillingly and unwittingly, yet by the very nature of the case he will assuredly be necessitated both consciously and voluntarily to perjure himself. Thus, oftentimes when we are dining at home, and one of the servants happens to do amiss, the wife swears that he shall be flogged, and then the husband swears the contrary, resisting, and not permitting it.
In this case, whatever they may do, perjury must in any case be the result; for however much they may wish and endeavour to keep their oaths, it is no longer possible; but whatever happens, one or other of these will be ensnared in perjury; or rather both in any case.
Source: Homilies on the Statues (New Advent)