Looking at digit sums: 10→1, 14→5, 16→7, 18→9, 21→3, 24→6, 26→8. The pattern is 1,5,7,9,6,8 - which are mostly +2 increments (1→3→5→7→9). The sequence should be 10,12,14,16,18,21,24,26. The number 21 breaks the +2 pattern in digit sums (should be around 20 for digit sum 2, or 30 for digit sum 3).