There's been some discussion recently about what is a "scam" and what is "Americans expecting business practices to be exactly the same as they are at home".
So here are the restaurant "scams" that my Routard guide to Paris warns its French readers about:
Cheap set menus or formules offered on the boards outside the restaurant that are mysteriously not present on the menu you are given once inside: you've probably heard of or encountered this already. Routard recommends that you always challenge this and walk out if the results are unsatisfactory. This may be a warning sign of other dishonesties or poor quality. Also beware of time limitations on formules which may be in very small print.
False claims that dishes are prepared from scratch when they are actually bought pre-cooked. "Fait maison" is not currently a legally-defined term, although there is a campaign to make it so.
High prices on branded mineral water: don't be afraid to assert your right to free tap water as aggressively as is necessary.
Wine prices quoted as half-bottles rather than full bottles with the half-bottle announcement in small print. I've personally never encountered this, but presumably if they mention it it can happen.
Wine vintage substitution with a highly-regarded vintage on the wine list but a less-popular vintage given to you. I don't order really expensive wines where the vintage matters, but if you do have a close look at the bottle label for the vintage and not just the wine name.
Cover charges for babes in arms that don't eat. According to Routard this is technically legal, but no decent restauranteur would actually try it nowadays. Again a potential warning sign of other bad behaviour, although it may not appear until the final bill.
The "submarine". This is a notorious petty scam practiced by waiters, where the coins given in change are left underneath the banknotes and/or receipt in the hope that the customer won't notice and will leave them behind. Can be avoided with just a little observation.
On another issue that sometimes gets mentioned here, the Routard writers note that it is technically illegal for a restaurant to refuse to serve a customer on the grounds that the order is too small. However they also point out that it is highly inconsiderate to the restauranteur to occupy a table at a busy time and only order a single main course between three people. If you genuinely have a very small appetite, it would be polite to order one of the priciest things on the wine list :-)