Since I previously said a little about Aquinas on lying, I thought I would add a little bit about Scotus on lying.
All the scholastics see the Ten Commandments as serving a sort of dual function: they identify or express essential elements of natural law but do so in a form specifically to be implemented in the covenant with Israel. The prohibition against false witness is, of course, one of the Ten Commandments, and was seen as expressing a natural-law prohibition against lying. All the scholastics are essentially agreed on this: lying is always wrong. They are also essentially agreed on most of the reasons for this: the authority of Scripture, the authority of Augustine, and the authority of reason (in the form of arguments that were universally considered at least probable, e.g., the fact that encouraging lying is bad for society) concurred on the point: that's a scholastic Triple Crown. There were some differences, however, and this was due to a key split in scholastic views on one important concept in natural law theory: dispensation. This is a very tricky issue, so what I say about it here will be brief and crude.
The best known account of dispensation of natural law today is Aquinas's. 'Natural law' covers two kinds of precepts: general precepts and precepts for particular situations. In precepts for particular situations the appropriate authority -- and especially God -- can grant a dispensation. For instance, usually you have to pay back loans, but God could in principle overrule that. The general precepts are exceptionless: nobody, not even God can actually dispense from them (because that would involve a contradiction). And Aquinas is very clear that all the precepts indicated by the Ten Commandments are general precepts.
There is another view, which in part due to Bonaventure, became associated with the Franciscans. On this view, only the precepts of the first table (which have to do with rendering what is due to God) are indispensable general precepts. The precepts associated with the second table (which have to do with rendering what is due to our neighbor) are dispensable by God. Scotus notes that the second-table commandments do not concern themselves with good as such, which the first-table commandments do, but good under various qualifications relevant to human beings. This means that in principle God could remove a condition implicit in the precept so that in principle they could be broken without compromising one's pursuit of the last end. The 'in principle' is important: Scotus goes on to argue tht while these dispensable precepts have no strictly necessary connection to the indispensable precepts, they have a very profound harmony with them, and so any such dispensation would necessarily be quite rare and for extraordinary purposes.
Thus when Aquinas says that lying is always wrong, he means there are absolutely no possible exceptions. When Scotus says that lying is always wrong, however, he wants to allow the possibility of a dispensation from God for extraordinary purposes. Any Scotists reading this can correct me, but as far as I know there's no evidence to think that Scotus holds that God has ever dispensed from the precept against lying -- there would have to be clear reason to think that God actually revealed such a dispensation. Dispensation from the precept against lying looks to me like a pure hypothetical for Scotus. But even if not, it is, again, rare, for extraordinary purposes, and would require clear signs from God. The only real difference between Aquinas and Scotus on this point is that Aquinas holds tht it is never possible to lie without doing something wrong, while Scotus would allow that it might be possible to do everything one does in a lie and yet not do something wrong if God had reason to set it up things in that case in a way that he generally (for good reason) does not. Either of these views can reasonably be held by a Catholic; but Scotus's view provides about as much leeway as can seriously be granted on the subject.
[Incidentally, on what it is about lying in itself that makes it wrong, the Wadding editors suggest that Scotus rejects Aquinas's view and accepts a different view. But, having read all the relevant passages in grad school while trying to pull together a paper on dispensation in natural law theory, and having re-read Scotus on lying, I don't see much similarity between Aquinas's actual view and the one Scotus rejects, and while the emphases are different, with, e.g., Aquinas putting more emphasis on the object in building his argument, every major element of the view Aquinas reaches seems to me to be in the view Scotus reaches and vice versa. Because their vocabulary is not exactly the same one could read them as putting forward essentially different views, but this doesn't seem obviously required by any argument made, and I think this is one of those areas where the differences between the Common Doctor and the Subtle Doctor have been exaggerated. At least, any assumption that they are opposed needs to be closely examined. But this is a side issue.]
