THE other evening I was walking along a dark York street when I spotted a group of about half a dozen teenagers, boys and girls, in a bus shelter. As I approached one of the girls shrieked: “No, stop!” while raising her arms, apparently in self-defence, as a boy loomed over her.

What to do? Rush right in, shout for help or maybe call the cops? Or could I be witnessing some kind of horseplay, maybe between a boyfriend and girlfriend? I decided to get a closer look.

It was a good job I did, because it turned out both my assumptions about what might be happening were completely wrong.

The girl was enthusiastically relating a story to her female friends. Her tale appeared to be about her shouting and waving her arms at a previous passing bus to try to get it to stop – without success.

As for the boys, they weren’t looming over anyone. They were actually standing a few feet from the girls, quietly listening. My mistake was, quite a literally, a failure of perspective.

Why did I jump to my initial, alarmist conclusions? Screams on a dark night is a fairly obvious clue.

It could be argued, I suppose, that while it might have been a minor disaster if I’d gone charging into that bus stop with all the wrong ideas, it was a genuine tragedy that people who heard screams near Joanna Yeates’s Bristol flat jumped to the opposite conclusion; that it would just be “kids” messing around.

Both cases are perhaps examples of why first impressions aren’t always right, and books can’t always be judged on their covers. Pretty obvious stuff, I guess, but if that’s the case, why do we keep doing it?

One of the mantras preached to young journalists is “never assume”, but instant assumptions are notoriously rife in journalism, and indeed in the whole of the modern mass media, politics and public life.

And that’s maybe just a reflection of human behaviour. My heart sinks when I hear people say their first impression of someone is always right; I suspect it only seems that way because they never allow themselves that second or third look that could prove them wrong.

How many job interviews are settled by initial responses rather than comparing CVs or all the tests candidates are called on to do?

How many miscarriages of justice have come about because a police officer was convinced they’d got their villain when they first clapped eyes on a particular suspect?

Mind you, instant judgements do work for some people. A woman once told me she judged potential dates on the quality of their footwear; she was particularly intrigued by one new acquaintance who had “very nice shoes”.

They’re still married, so maybe there’s something to first impressions after all.

• THOUSANDS of heartbroken Welsh rugby fans are convinced it was a referee’s false first impression that smashed their World Cup hopes when skipper Sam Warburton was sent off on Saturday.

Trouble is, to my eye Warburton’s tackle actually looked worse in a slow motion repeat, and refs were told before the tournament to wave red cards in certain circumstances – including “tip tackles”.

Still, you’d have thought a yellow card was enough.

Despite the red card, France were so poor Wales could have snatched the game had they shown a bit more composure. It was a sad end for the best European team in the competition.

No one wears rugby martyrdom more obsessively than Welsh fans, so we can look forward to years of hearing how their boys would have won the cup but for that referee – notwithstanding the fact their team hasn’t beaten the All Blacks in more than half a century.

Two consolations – imagine how unbearable those same fans would have been if Wales had reached the final, and thank goodness the ref wasn’t English.