They had forgotten ( ἐπελάθοντο, not pluperfect); came to the other side, and forgot; obliti sunt (Vulgate); i.e. they perceived that they had forgotten to take sufficient bread for the journey before them.
The district which they were about to traverse was but sparsely inhabited, and offered no hope of supplying this want. It is doubted whether the ensuing conversation took place during the voyage or after they had landed.
The language of St. Mark inclines one to believe that the deficiency was discovered during the transit, and the remarks now narrated were made then. As it would take some hours to cross, there was ample time to feel and expatiate upon the need; and if Christ had told them of his future movements, they would naturally feel regret for their carelessness and want of forethought.
Or it might be that Christ's observation concerning the leaven was made in the beat, and his reproof of their thoughts was given on landing.