In English, 'preservatives' are substances that we use to stop products decaying. For example, a manufactured drink or food product may contain preservatives so that it lasts longer and you don't need to keep it in the fridge.

In French, 'préservatifs' are condoms. Could be embarrassing.

sympathetic / sympathique

- sympathique en Francais = pleasant, friendly

exhibition in English = exposition en Francais 

- exhibition en Francais = flashing as in indecent exposure of your naked body 

Here are a few false friends I could think of:

1. to pretend = faire semblant

    prétendre = to claim

2. a coin = une pièce

    un coin = a corner

3. pain = douleur

    pain = bread

4. a library = une bibliothèque

     une librairie = a book shop

5. to pass an exam = réussir un examen

    passer un examen = to take an exam

6. a hazard = un danger

    le hasard = chance

7. a physician = un médecin

    un physicien = a physicist

8. actually = en fait

    actuellement = currently

french : Actuallement = currently

english : Actually = in fact

The English word "sensible" is a false friend for all Romance language native speakers, including the French.  In English, it means having good sense, reasonable, practical.  In French, "sensible" = sensitive, which has a different meaning in English.  

I think "sensible" also meant sensitive in English in the past, and this meaning lives on in the noun "sensibility", which readers of Jane Austen will be familiar with.