I recently had need to make a return journey on my Oyster card from
Kings Cross St Pancras tube station to the Canonbury Overground station.

Interchanging at Highbury and Islington on the outward journey there was
a pink Oyster validator on the Overground platform at Highbury and
Islington with instructions that one must touch the validator when
interchanging at that station, and a notice about penalties for
travelling without a validated Oyster card. It was worded in a way that
didn't seem to make touching the pink validator optional.

My recollection of previous descriptions on this group was that the pink
validators were route validators, and that one could optionally touch
them to get cheaper journeys when one took a route using fewer zones?

So, is their use really compulsory? If so, under what circumstances?
Does one always need to check for route validators on every platform one
interchanges at, or is Underground-to-Overground transfer a special case?

I touched the pink validitor as instructed (although, not expecting it,
I very nearly failed to notice the validator); what (if anything) would
have happened if I *hadn't* touched the pink validator?

Thanks,

-roy