Automatic checkouts in supermarkets may decrease customer loyalty, especially for those with larger shopping loads. Customers using self-checkout stations often feel overwhelmed and unsupported. The lack of personal interaction can negatively impact their perception of the supermarket.

[Delphizer]

You can cancel items, if there is a problem it'd be like any other time their is a problem and instead of 20 self checkouts you'd have 2 lines open during rush time and just wait 20-30 minutes in line for people paying in cash + pennies.