Attempting to add new DNS records to a given DNS zone results in errors similar to the following:
Error: API failure: Zone is invalid: Line XX: ignoring out-of-zone data (domain.tld) at /usr/local/cpanel/Cpanel/ZoneFile/LineEdit.pm line 390
DNS zones can only hold records for the domain the zone is for. Attempting to add records for a different domain will result in the above error. The error references the line with the issue and gives a hint as to what the referenced domain is.
Remove the record on the line given in the DNS zone. The zone will not be loaded until the record is removed.