When someone spilled red wine on my dress, I grabbed a glass of white wine and ran to the ladies'. Almost everyone I passed on the way shrieked "throw white wine on it, it'll get the stain out!". Hello – that's why I had a glass of it in my hand.

But when I took it to the dry cleaner's, the lady behind the counter said: "So we've now got two stains to get out of your dress instead of one."

"Shouldn't I have done it?" I asked. "No," she said.

I couldn't believe it. I was certain you should throw white wine on a red wine stain. I can't remember where I first heard it, it's just something I knew, or thought I knew. And I've told others to do it over the years ...

It's astonishing that in our questioning age when some of us are too suspicious to even open our front door if we aren't expecting visitors, we still accept some things we're told as fact and pass them on without question.

Friends all over the world still ask if I've heard about Bristol Zoo's car park attendant pocketing decades worth of parking fees, only for me to explain it's a complete urban myth.

But some are dangerous. This week, a friend emailed everyone she knows warning that a kidnapped girl was found in the toilets of a major supermarket (which she named) with two women shaving the girl's head and dressing her in boys' clothes. My friend urged us all to forward the message to everyone we know. I Googled it – and found reports from across the country exposing it as a cruel internet hoax.

My friend passed it on in good faith, but if she'd only done a quick internet search she'd have spared herself, her friends, their friends, and so on, unnecessary worry.

So who started it? Nobody knows. A disgruntled ex-exployee wanting to blacken the supermarket chain's name and scare away its customers, perhaps?

Years ago at work, we all got an email warning that our computer had been infected by a virus and we needed to take certain steps to avoid a major meltdown. I rang our IT department who said "it's a hoax – the instructions to 'fix' it actually let a virus in".

I told my friend Jo and her face went white. She'd already followed the instructions, clicking on particular files and typing in certain commands. And she wasn't the only one.

But our cynicism prompted by such fakery is itself potentially dangerous. National newspapers reported on a recent study which claims that women having their drinks spiked and being sexually attacked is an urban myth dreamt up by binge-drinking floozies ashamed to admit the truth.

This could endanger women by making them less vigilant about leaving drinks unattended.

Not only that, but what impact could it have on genuine victims?