When you type an entry in a ComboBox control Access will attempt to complete the entry based on the control's lookup list. This is controlled by the AutoExpand property, which is set to "Yes" by default.
If your value list contains several items that are close in spelling, it is easy for users to let Access choose the wrong item by accident.
You can avoid errors by setting the control's AutoExpand property to "No" in Design view.
Once the change has been made, users will be forced to type the entire entry or select an item using the ComboBox control's dropdown list.