Faux Amis

There are lots of words in French that are the same as, or very similar to, English words. You just have to say it with a French accent and you've got it. Words like address, finance, expression, place, fracture, helicopter, village, and tunnel all work like that. But you have to be careful because there are some words that sound the same but have quite a different meaning. These are called false friends or, in French, "faux amis". Occasion is one of these, which means "second-hand" or "rented" in French.
I discovered another faux ami when I was eating a friend's delicious tiramisu recently. I tried to say that home-made is nicer than shop-bought because it doesn't taste like preservatives. I tried the easy way out by saying preservative with a French accent. But by the look on their faces I knew I had made a big mistake. See if you can work out just how big by looking at the clues in the photo below...

A vending machine outside a pharmacy

P.S.
I think the French word I should have used is "conservateurs", but if you try it on Yahoo Babelfish translation service, you will find it makes the same mistake!