I read on the Wikipedia about the “Gimli Glider” incident. Apparently it was a culmination of multiple errors. The way I understand it, the Boeing 767 had two redundant fuel gauges. Howver, it was a new craft and they were still working out the kinks. On this particular 767, the fuel gauge wouldn't show the correct level if both were function, so the mechanic pulled the breaker on one of the gauges. Next, a mechanic at the next stop reset the breaker and forgot to notate it. Also, they were using the wrong conversion factor between kilograms and liters of fuel. They used the conversion factor between pounds and gallons. So they had around half what they thought they did. You can see what's about to happen. The plane runs out of fuel at 41,000 feet. The pilots then glide the aircraft to a military airstrip that was at the time set up for a “Family Day” drag race. They landed nevertheless. The van of mechanics sent from the nearest airport to the impromptu landing, coincidentally, were delayed when the van ran out of fuel.
A similar case involved an Airbus A310. It couldn't raise it's landing gear all the way, and it's computers calculated fuel consumption as if the landing gear were up. It ran out of fuel 20 km away from it's destination. They glided it to the runway, landed short and totaled the plane. No one died.
Sometimes nature can turn a powered craft into an unpowered glider. A Boeing 747 flying near Java flew through a volcanic ash cloud. Engines, it turns out, don't like being flown through volcanic ash clouds. In protest, they flamed out. The pilots had to glide 261 km. Being British, they announced this as “Ladies and Gentlemen, this is your Captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress.” I wouldn't tell the story if they crashed, so you know the ending. Why did they fly through an ash cloud? Because it didn't show up on their weather radar. Thanks, computers.
Our final story of “planes that really shouldn't be used as gliders” is a Airbus A330. This time, the mechanic “upgraded” a hydraulic line, but use the wrong part. He told his boss it wasn't the right part, but he said to use it anyway. This left not enough room between the hydraulic and fuel lines. The hydraulic line vibrated enough to cause a fuel leak in the fuel line. The pilots, while over the Atlantic, noticed a fuel imbalance between the two tanks, so they opened the cross-feed valve. This just poured their fuel from the good tank to the leaky tank. Consequently, they ran out of fuel and had to glide to an airbase in the Azores. What I want you to do is find a map and find the Azores. Notice how much nothing there is surrounding them? They were pretty lucky they were within distance. Not much to land on in an ocean.
All information courtesy of Wikipedia: