When you type an entry in a combobox control Access will typically attempt to complete the entry based on the control's lookup list.
This is controlled by the AutoExpand property, which is set to Yes (-1) by default.
Although such behavior is helpful, it can cause problems if your value list contains several items that are close in spelling, since it's easy for users to accidentally let Access choose the wrong item.
You can avoid errors by setting the control's AutoExpand property to No (0) in Design view or using VBA to set the property equal to 0.
Once you've made the change users are forced to type the entire entry or select an item using the combobox control's dropdown list.