The Church Fathers provide evidence

The Fathers provide evidence both that manuscripts differed and that copyists could introduce errors; however, they did not all describe these differences specifically as “many unintentional copying errors.”

Direct patristic evidence

• Origen complained that differences among Gospel manuscripts had become considerable, attributing them to scribal carelessness, excessive boldness in altering texts, or inadequate correction. He wrote that “the differences among the manuscripts have become great,” naming “the negligence of some copyists” among the causes.[biblequestions]
• Jerome likewise discussed divergent Latin biblical copies and undertook revision by comparison with Greek manuscripts, reflecting an awareness that transmission had produced conflicting readings.[biblequestions]
• Tertullian accused opponents of corrupting or adulterating biblical texts, showing that early Christians could distinguish a disputed textual form from what they considered the authentic one.[newadvent]

Important qualification

The Fathers’ comments establish that textual variation was already visible in antiquity, and that at least some writers blamed ordinary copying negligence. But they also attributed some variants to deliberate alteration, especially by theological opponents; Tertullian is a prominent example.[newadvent][biblequestions]

So a careful formulation would be:

Church Fathers such as Origen and Jerome recognized substantial variation among biblical manuscripts. Origen explicitly included copyists’ negligence among its causes, while other Fathers also alleged intentional textual alteration by opponents.[biblequestions][newadvent]

That is stronger historically than claiming the Fathers uniformly said that “many” variants were unintentional, since their surviving remarks address both accidental and intentional causes.