How to resolve error: “An unexpected error has occurred” when saving a custom field.

User is getting the “An unexpected error has occurred.” when saving a custom field.

Possible reason is because a non-numeric character was found where a numeric was expected to be a value on Custom Field. 

steps:

1. Navigate to Customization > Lists, Record & Fields > Transactions Body Fields
2. Look for Custom Field 
3. Click Edit
4. Go to Validation & Defaulting tab
5. Default Value: Remove Value
6. Formula: Remove Checkmark
7. Click Save