I bought a navy tennis skirt in San Francisco and experienced extreme post-purchase dissonance back at the hotel where I discovered the outfit didn't make me look miraculously trim and toned after all. It had been the height of optimism in the first place to think a mere power skirt could dissolve stubborn pounds. This was the day I discovered that mirrors do, in fact, sometimes lie - and that wonky, distorting mirrors are not confined to the mirror halls at funfairs.

In San Fran I decided I'd been duped by a skinny mirror and so I high-tailed it back to the store to inspect the mirrors in question. They were fixed to the wall and I thought I could perceive a subtle tilt of the glass towards the person being reflected. It looked like a full-length wooden wedge-shape had been inserted between the wall and the mirror.

"A-ha! I've been had by the old tilted mirror trick," I thought as I closely examined both edges of the mirror and then my unusually flattering reflection.

The staff members must have thought I was mad.

My theory, of course, was that these mirrors had been installed to appeal to the shopper's sense of vanity and to give the impression that the store's clothes dramatically enhanced their body shape. I couldn't believe there wasn't a law against it. Yet it's clearly not a universal strategy since someone recently tweeted: "Congrats, Niketown. Your dressing room mirrors make me want to find the closest circuit trainer."

Nonetheless affronted by the duplicity, I spent a few years following that incident minutely inspecting mirrors in changing rooms. If they looked like they had a bit of a lean on - or had been otherwise tampered with - I didn't even try on the clothes. I had no desire to be taken in by a cynical retail strategy designed to boost sales. Oddly enough there are some people who actually like slimming mirrors and the temporary boost they give to the ego but leaving the store and ultimately facing reality must be a bit of a downer.

There's only one thing worse than skinny mirrors in shop changing rooms and that's having no mirrors. I've lost count of the upmarket stores I've encountered that expect you to schlep out of the changing room and parade in front of a communal mirror. I frequently visited the Karen Walker store in Newmarket and always made a point of saying in my most disappointed tone, "Oh, still no mirrors in the changing rooms then." (Okay, there was a mirror but it was high on the wall and really only large enough for touching up your makeup.)

I decided that shops without mirrors were silently signalling the message that if you weren't relaxed enough to check out your clothes in view of the public then you weren't a person worthy of wearing the brand. That theory was discredited when after many years full-length mirror were installed in the changing rooms at Karen Walker. Perhaps they'd finally realised that there's a significant subset of women who aren't prepared to be seen in a garment before they've had a chance to assess how it looks. Or maybe my nagging wasn't as impotent as it had seemed at the time.

What's your opinion of skinny mirrors in stores? Have you ever encountered one? Do you loathe them or love them? And how do you feel about stores that have no mirrors in their changing rooms?