Take the tale of one poor sap who slipped a ring into a creme brulee:
Smash went the crust. In went the spoon. And before Lopes could say, “Um, I have something to ask you,” his brilliant-cut one-carat surprise went sliding down his intended’s throat. “Our first hug was the Heimlich maneuver,” he recalls. “My advice to a man about to propose is: Use creativity only up to a point. You don’t want your girlfriend to end up in the hospital on her engagement night.”
Beyond the health hazards (and one restaurant has devised "a special plate with a trapdoor and a secret compartment" to avoid these scenarios) there are the less dramatic drawbacks: someone not finishing a dish, so that a ring has to be anticlimactically fished out of the food, or the cases when a woman doesn't feel like ordering her "favorite" dessert, so that an unfortunate waiter is in the position of pressuring her.
Gourmet offers a series of tips to avoid proposal mishaps, several of which are kind of depressing: sit in the center of the room (since presumably anyone making such a public proposal "wants to be seen") and tell your mother-in-law in advance so she'll pick up the tab. Romantic! Oh, and in case you were getting too sentimental, apparently restaurants are just as used to seeing "divorce dinners" as proposals. Fun fact: February 13th is when guys take their mistresses out; they have to save the real thing for their wives. Happy Valentine's Day! Remind me to eat in. [Lovin’ Spoonfuls]