As long as we're talking about wrinkles in Pseudo-Isidorian reception, we might as well bring up the curious case of Pseudo-Isidore and the Consecration of Chrism
Way back in 2010, we took a look at the thirty-sixth forged decretal (item 42): Pope Fabian to all Eastern bishops and faithful, and we noticed that the letter insists that bishops everywhere consecrate chrism annually--presumably because only bishops could consecrate chrism and the entire matter redounds to issues of episcopal authority. Usually Pseudo-Isidore ascribes liturgical prescriptions to his popes when he is encouraged to do so by the Liber Pontificalis. In this case, though, the LP biography for Fabian mentions chrism not at all. And the passage in Pseudo-Fabian is long and quite emphatic--a rare paragraph-length chunk of text that appears to be an entirely original composition of our forgers.
At the time, we noted that chrism isn't really a major concern for Pseudo-Isidore. As far as I know, the forgeries address chrism on only one other occasion. That is at Benedictus Levita 3.394--a forged capitulum that forbids "priests and their ministers" from seeking chrism from anyone except their own bishops, and that proceeds to forbid anyone from consecrating chrism on any other day of the year beyond Holy (or Maundy) Thursday.
As with Fabian, there are no sources in sight for this latter prohibition. In fact, as Wilfried Hartmann once noted, the Benedictus capitulum directly contradicts canons promulgated at Toledo I and Braga II, which allow bishops to consecrate chrism whenever they're in the mood. What's more, Part II of Pseudo-Isidore transmits both of these canons completely unmolested, exactly as they occur in the Hispana Gallica. Were our forgers asleep at the wheel? Or was their interest in chrism a later, post-Hispana development?
These questions are hard to answer, and there are still harder ones. In 845, bishops gathered at the Council of Meaux/Paris promulgated one canon, namely the 46th, bluntly declaring that "no one should presume...to consecrate chrism," except on Holy Thursday. The laconic canon cites no authorities or precedents, but as Emil Seckel noted, the substance, if not the words, are directly congruent with BL 3.394.
In other words, in 845, the Council of Meaux/Paris legislates a unique point of law otherwise found only in Benedictus Levita. This is the same year that Ebo of Reims forges a privilege in the name of Pope Gregory IV that similarly alludes to (what we normally think of as) Pseudo-Isidorian theories about episcopal transmigration and the exceptio spolii, and similarly cites no authorities or precedents.