OK  debug shows, that IBSQL contains code:

SQLExecProcedure:
    begin
      fetch_res := Call(FGDSLibrary.isc_dsql_execute2(StatusVector, TRHandle,
                            @FHandle, Database.SQLDialect, FSQLParams.AsXSQLDA,
                            FSQLRecord.AsXSQLDA), False);
      if (fetch_res <> 0) then
      begin
        if (fetch_res <> isc_lock_conflict) then
        begin
           { Sometimes a prepared stored procedure appears to get
             off sync on the server ....This code is meant to try
             to work around the problem simply by "retrying". This
             need to be reproduced and fixed.
           }
          FGDSLibrary.isc_dsql_prepare(StatusVector, TRHandle, @FHandle, 0,
                          PByte(FProcessedSQL.Text), Database.SQLDialect, nil);
          Call(FGDSLibrary.isc_dsql_execute2(StatusVector, TRHandle,
                             @FHandle, Database.SQLDialect, FSQLParams.AsXSQLDA,
                             FSQLRecord.AsXSQLDA), True);
        end
        else
          IBDataBaseError;  // go ahead and raise the lock conflict
      end;
    end

and the error message about unprepared statement is raisex exactly upon the second execution isc_dsql_execute2(...), so - maybe such second try is not necessary and we can raise Exception IBDataBaseError whenever the fetch_res is not 0?