Custom fields and save site as template problem

If you have custom fields with custom properties on your site and use default field serialization you'll get an error while saving such site as template. It happens because of template validation through XSD-scheme 14\TEMPLATE\XML\wss.xsd. Let's see how it looks:

<xs:complexType name="FieldDefinition" mixed="true">
    <xs:all>
      <xs:element name="FieldRefs" type="FieldRefDefinitions" minOccurs="0" maxOccurs="1" />
      <xs:element name="DisplayPattern" type="CamlViewRoot" minOccurs="0" maxOccurs="1" />
      <xs:element name="DisplayBidiPattern" type="CamlViewRoot" minOccurs="0" maxOccurs="1" />
      <xs:element name="CHOICES" type="CHOICEDEFINITIONS" minOccurs="0" maxOccurs="1" />
      <xs:element name="MAPPINGS" type="MAPPINGDEFINITIONS" minOccurs="0" maxOccurs="1" />
      <xs:element name="Default" type="xs:string" minOccurs="0" maxOccurs="1" />
      <xs:element name="Formula" type="xs:string" minOccurs="0" maxOccurs="1" />
      <xs:element name="FormulaDisplayNames" type="xs:string" minOccurs="0" maxOccurs="1" />
      <xs:element name="DefaultFormula" type="xs:string" minOccurs="0" maxOccurs="1" />
      <xs:element name="Validation" type="ValidationDefinition" minOccurs="0" maxOccurs="1" />
      <xs:element name="Customization" type="CustomizationDefinition" minOccurs="0" maxOccurs="1" />
    </xs:all>
    <xs:attribute name="Aggregation" type="xs:string" />
    <xs:attribute name="AllowDeletion" type="TRUEFALSE" />
    <xs:attribute name="AllowDuplicateValues" type="TRUEFALSE" />
    <xs:attribute name="AllowHyperlink" type="TRUEFALSE" />
    <xs:attribute name="AllowMultiVote" type="TRUEFALSE" />
    <xs:attribute name="AppendOnly" type="TRUEFALSE" />
    <xs:attribute name="AuthoringInfo" type="xs:string" />
    <xs:attribute name="BaseType" type="xs:string" />
    <xs:attribute name="CalType" type="xs:int" />
    <xs:attribute name="CanToggleHidden" type="TRUEFALSE" />
    <xs:attribute name="ClassInfo" type="xs:string" />
    <xs:attribute name="ColName" type="xs:string" /> 
    <xs:attribute name="ColName2" type="xs:string" use="optional" />
    <xs:attribute name="Commas" type="TRUEFALSE" />
    <xs:attribute name="Customization" type="xs:string" />
    <xs:attribute name="DefaultListField" type="TRUEFALSE" />
    <xs:attribute name="Decimals" type="xs:int" />
    <xs:attribute name="Description" type="xs:string" />
    <xs:attribute name="Dir" type="xs:string" />
    <xs:attribute name="Direction" type="xs:string" />
    <xs:attribute name="DisplayImage" type="xs:string" />
    <xs:attribute name="DisplayName" type="xs:string" />
    <xs:attribute name="DisplayNameSrcField" type="xs:string" />
    <xs:attribute name="DisplaySize" type="xs:int" />
    <xs:attribute name="Div" type="xs:string" />
    <xs:attribute name="ListItemMenu" type="TRUEFALSE" />
    <xs:attribute name="ListItemMenuAllowed" type="ListItemMenuType" />
    <xs:attribute name="EnableLookup" type="TRUEFALSE" />
    <xs:attribute name="EnforceUniqueValues" type="TRUEFALSE" />
    <xs:attribute name="ExceptionImage" type="xs:string" />
    <xs:attribute name="FieldRef" type="xs:string" />
    <xs:attribute name="FillInChoice" type="TRUEFALSE" />
    <xs:attribute name="Filterable" type="TRUEFALSE" />
    <xs:attribute name="FilterableNoRecurrence" type="TRUEFALSE" />
    <xs:attribute name="ForcedDisplay" type="xs:string" />
    <xs:attribute name="ForcePromoteDemote" type="TRUEFALSE" />
    <xs:attribute name="Format" type="DisplayFormat" />
    <xs:attribute name="FromBaseType" type="TRUEFALSE" />
    <xs:attribute name="HeaderImage" type="xs:string" />
    <xs:attribute name="Height" type="xs:int" />
    <xs:attribute name="Hidden" type="TRUEFALSE" />
    <xs:attribute name="HTMLEncode" type="xs:string" />
    <xs:attribute name="Id" type="xs:string" />
    <xs:attribute name="IMEMode" type="IMEMode" />
    <xs:attribute name="Indexed" type="TRUEFALSE" />
    <xs:attribute name="IsolateStyles" type="TRUEFALSE" />
    <xs:attribute name="IsRelationship" type="TRUEFALSE" />
    <xs:attribute name="JoinColName" type="xs:string" />
    <xs:attribute name="JoinType" type="JoinType" />
    <xs:attribute name="LCID" type="LocalizableString" />
    <xs:attribute name="LinkToItem" type="TRUEFALSE" />
    <xs:attribute name="LinkToItemAllowed" type="ListItemMenuType" />
    <xs:attribute name="List" type="xs:string" />
    <xs:attribute name="Max" type="xs:float" />
    <xs:attribute name="Min" type="xs:string" />
    <xs:attribute name="Mult" type="TRUEFALSE" />
    <xs:attribute name="Name" type="xs:string" use="required"/>
    <xs:attribute name="NegativeFormat" type="xs:string" />
    <xs:attribute name="Node" type="xs:string" />
    <xs:attribute name="NoEditFormBreak" type="TRUEFALSE" />
    <xs:attribute name="NumLines" type="xs:int" />
    <xs:attribute name="Percentage" type="TRUEFALSE" />
    <xs:attribute name="PIAttribute" type="xs:string" />
    <xs:attribute name="PITarget" type="xs:string" />
    <xs:attribute name="PrimaryPIAttribute" type="xs:string" />
    <xs:attribute name="PrimaryPITarget" type="xs:string" />
    <xs:attribute name="Presence" type="TRUEFALSE" />
    <xs:attribute name="PrimaryKey" type="TRUEFALSE" />
    <xs:attribute name="ReadOnly" type="TRUEFALSE" />
    <xs:attribute name="ReadOnlyEnforced" type="TRUEFALSE" />
    <xs:attribute name="RelationshipDeleteBehavior" type="DeleteBehaviorType" />
    <xs:attribute name="RenderXMLUsingPattern" type="TRUEFALSE" />
    <xs:attribute name="Required" type="TRUEFALSE" />
    <xs:attribute name="RestrictedMode" type="TRUEFALSE" />
    <xs:attribute name="ResultType" type="xs:string" />
    <xs:attribute name="RichTextMode" type="xs:string" />
    <xs:attribute name="RichText" type="TRUEFALSE" />
    <xs:attribute name="Sealed" type="TRUEFALSE" />
    <xs:attribute name="SeperateLine" type="TRUEFALSE" />
    <xs:attribute name="SetAs" type="xs:string" />
    <xs:attribute name="ShowAddressBookButton" type="TRUEFALSE" />
    <xs:attribute name="ShowAlways" type="TRUEFALSE" />
    <xs:attribute name="ShowField" type="xs:string" />
    <xs:attribute name="ShowInDisplayForm" type="TRUEFALSE" />
    <xs:attribute name="ShowInEditForm" type="TRUEFALSE" />
    <xs:attribute name="ShowInFileDlg" type="TRUEFALSE" />
    <xs:attribute name="ShowInListSettings" type="TRUEFALSE" />
    <xs:attribute name="ShowInNewForm" type="TRUEFALSE" />
    <xs:attribute name="ShowInViewForms" type="TRUEFALSE" />
    <xs:attribute name="Sortable" type="TRUEFALSE" />
    <xs:attribute name="StorageTZ" type="xs:string" />
    <xs:attribute name="StripWS" type="xs:string" />
    <xs:attribute name="SuppressNameDisplay" type="TRUEFALSE" />
    <xs:attribute name="TextOnly" type="TRUEFALSE" />
    <xs:attribute name="Title" type="xs:string" />
    <xs:attribute name="Type" type="xs:string" use="required" />
    <xs:attribute name="UniqueId" type="xs:string" />
    <xs:attribute name="UnlimitedLengthInDocumentLibrary" type="TRUEFALSE" />
    <xs:attribute name="URLEncode" type="TRUEFALSE" />
    <xs:attribute name="URLEncodeAsURL" type="TRUEFALSE" />
    <xs:attribute name="Version" type="xs:string" use="optional" />
    <xs:attribute name="Viewable" type="TRUEFALSE" />
    <xs:attribute name="WikiLinking" type="TRUEFALSE" />
    <xs:attribute name="Width" type="xs:int" />
    <xs:attribute name="WebId" type="UniqueIdentifier" use="optional" />
    <xs:attribute name="XName" type="xs:string" />
    <xs:attribute name="ID" type="UniqueIdentifier" />
    <xs:attribute name="Group" type="xs:string" />
    <xs:attribute name="MaxLength" type="xs:int" />
    <xs:attribute name="SourceID" type="xs:string" />
    <xs:attribute name="StaticName" type="xs:string" />
    <xs:attribute name="JoinRowOrdinal" type="xs:int" />
    <xs:attribute name="RowOrdinal" type="xs:int" />
    <xs:attribute name="ShowInVersionHistory" type="TRUEFALSE" />
    <xs:attribute name="PrependId" type="TRUEFALSE" />
    <xs:attribute name="DisplaceOnUpgrade" type="TRUEFALSE" />
    <xs:attribute name="UserSelectionMode" type="xs:string" />
    <xs:attribute name="UserSelectionScope" type="xs:int" />
    <xs:anyAttribute namespace="##other" processContents="lax" />    
</xs:complexType>

If you use default namespace for your custom property and its name differs from properties defined in this file, you'll get validation error because of this attribute namespace="##other" of xs:anyAttribute element.

You can change schema to pass validation. Just remove an attribute namespace="##other" and you can create fields with any properties with default namespace.