When rebuilding the link database or rendering an item with a "general link" field, the following error can occur:
System.FormatException: Unrecognized Guid format. Actual value: ---> System.FormatException: Unrecognized Guid format.
at System.Guid.GuidResult.SetFailure(ParseFailureKind failure, String failureMessageID, Object failureMessageFormatArgument, String failureArgumentName, Exception innerException)
at System.Guid.TryParseGuid(String g, GuidStyles flags, GuidResult& result)
at System.Guid..ctor(String g)
at Sitecore.Data.ID..ctor(String id)
The error appears if some "general link" field (or deprecated "link" field) contains an empty "ID" attribute in its raw value, or doesn't contain it at all.