It wasn't really a shock that John and Sebastian would meet - I mean, they both vacate a home in London and they both don't have very reliable flatmates so of course they'd bang into each other when they were out shopping. John had been forced to leave the house to retrieve more milk and jam thanks to an accident on Sherlock's part which resulted in 6 week old eyeballs rolling around in both jar and jug. Sebastian had been forced to go to the shops for a similar reason, but his wasn't eyeball-related... his was more of the lead bullets embedded in the freezer at 4am that resulted in a complete loss of all frozen food that they had stored related.
John had been struggling to reach the shelf which now contained the strawberry jam as Sebastian moved over to the opposite side of the aisle to grab some mustard, both not meeting each other's gaze nor even noticing the others presence until a sudden slip of a certain doctors' shoe caused a trip backwards and a collosion of army veterans. Sebastian dropped the jar as the sudden noise of someone coming behind him made him jump into action, spinning around and catching John from behind before throwing him down onto his knees. He was used to being jumped by Jim but this was different, and once the adreneline rush stopped and the realisation of public eye kicked in Sebastian swallowed deep. John had had a similar rush of adreneline which had resulted in the cock of a gun and the aiming of a barrel at Sebastian's chest. His realisation kicked in a few seconds after Sebastians, causing him to swiftly move up from the ground he'd been thrown down on and pocketed the gun that he had almost let off.
The two men glanced at each other, embarrassment in both men's faces and shame in both of their eyes as they sized up each other. Sebastian knew exactly who this was, which made this even more awkward. John, on the other hand, simply saw a man twice his size whom he vaguely recognised from some long time ago... They didn't speak to each other, they didn't even apologise to each other but simply nodded in apology and then went back to what they were doing beforehand, completely ignoring the fact that they had both responded in identical ways. Completely oblivious to the fact that they had both ended up in the same supermarket under similar circumstances. Completely unknowledgable of the fact that they would both be seeing more of each other from then on.