A venial sin, for you acted without evil intention.
I did not think you would call these venial errors !
The faithful are bound to confess all "mortal" sins; they need not confess "venial" sins.
No fault was found in their life and teaching; but they were forbidden to define any sins as being mortal or venial until they had studied for four years.
The celebration is based on the doctrine that the souls of the faithful which at death have not been cleansed from venial sins, or have not atoned for past transgressions, cannot attain the Beatific Vision, and that they may be helped to do so by prayer and by the sacrifice of the mass.