1. In seaport towns, a huge number of American citizens would have had some knowledge of ship movements. A large convoy like the one for Operation Torch (the Alllied Invasion of North Africa) would have had over 100 ships and 35,000 troops sailing from Norfolk/Newport News. Local residents of the city would have been able to observe the sudden changes in shipping in the harbor, which is visible from many points in the Hampton Roads area, and to note the changes in army troop levels entering and "disappearing" from the city and be able to deduce it's time of sailing, a crucial element in finding it at sea. The safety of convoys like these were very much dependent on being hard to find at sea. If they were caught by a U-Boat "wolfpack" in 1942 or 43 they might suffer heavy losses, (although most were not located at all).

Of course, the idea of "loose lips sinking ships" was intended to apply to more than just ship movements. The graphic image of a sunken ship was a metaphor or example showing the effects that information passed could have on the war effort. With the American economy mobilized for war in most sectors, the there would have been vast swaths of the citizenry that would have known something at least slightly sensitive.

2. I can't think of an example of a specifically American example of such a sinking. However, that doesn't mean it did not happen. Japanese Naval Intelligence burned it's archives after the war and many Naval intelligence officers never discussed wartime activities with historians in their lifetime. Recent scholarship on IJN Intelligence suggests [they did have an intelligence ring that operated out of the Honolulu consulate before the war began.] (http://www.usni.org/magazines/navalhistory/2009-12/how-japanese-did-it) I don't think it's currently known if they received any wartime intelligence from Honolulu but it can't be ruled out. German Naval intelligence is known to have been surprisingly ineffective in WWII, but the fear of it was not without a basis in fact.

Lastly of course, American authorities knew that intelligence on shipping was valuable, because the American intelligence services went to considerable effort to obtain tactical intelligence on Axis sailings. Because Axis forces operated in occupied countries, they were more frequently successful. For example, "Coastwatchers" in the Pacific Theater were very effective in reporting Japanese Sea and air movements although their efforts were not primarily based around collecting "loose talk" so much as direct observation by friendly local populations.