It’s one of the unwritten rules of the Tartan Army – club allegiances are left at the turnstile when your ticket is handed over to get into a Scotland game.
The flouting of it by a number of supporters in Pittodrie on Thursday night was widespread enough to not be dismissed as the clichéd ‘tiny minority’.
Whether you think Ryan Jack committed a crime against his home-town club by signing for Rangers in the summer or you believe he did absolutely nothing wrong, a line was crossed.
More controversial characters than Jack have pulled on the dark blue of Scotland without being subjected to the treatment a young man making his debut had to endure in the Netherlands friendly.
Momentum is building behind a move away from Hampden Park for the national team but nights like this help support the case for staying at the national stadium.
Players get booed at Hampden, alright. Always have, always will.
Poor Chris Martin had jeers rather than cheers ringing in his ears when he was brought on against Slovenia and there have been plenty who have been singled out before him even when we were getting to championship finals, like Gary McAllister.
But as disappointing as the sound of a Scotland player trying his best (or in Martin’s case getting ready to try his best) getting stick en masse will always be, Jack’s case is different. For Martin, McAllister and the rest, fans were letting it be known what they thought of the player’s footballing ability or the manager’s faith in it.
With Jack it was all about domestic football.
What next? Jason Cummings targeted at Tynecastle? Kieran Tierney at Ibrox?
If being in your familiar Saturday surroundings makes casting aside your familiar club tribalism harder to achieve, perhaps taking Scotland games on the road isn’t the win-win many think it will be.