Very cute Gareth...

Will this happen every time you make one of these puzzles or was it a one-off effect?
Can this become a rule for those type of puzzles like my Samurai Flower Rule (Ninja-Leap)?

Yes - great puzzle Gareth! I must admit I wasted an hour not being careful enough on the diagonals (frustrating how one wrong number can muck up the complete puzzle but you don't realise until the very end!) However - wanting to see the effect you and Spittledung had mentioned I persevered and it all fell into place at the second attempt! Great puzzle - would love to see another of these.

Ditto, loved this puzzle. Messed up the first attempt, but got there the second time round, Great fun!