I've posted this story elsewhere for different reasons. In 2010 I was flying from Portland, OR to Amsterdam on Delta, then on to Finland. In Portland I was called up to the podium, told my seat was unavailable, but they found me a different seat -- in J. OK, this trip is starting off really well.
Well into the flight, after dinner, at which point I had turned down the hot fudge sundae, hoping to have it later, I noticed that we were flying low over Greenland, and heading east!. I discretely called over the flight attendant and asked what was going on? Pressurization problems and they were "hoping" to make it to Newfoundland because fuel was really low. Not a particularly reassuring way to say it.
Long story short, we spent 2 days in Happy Valley/Goose Bay, Newfoundland and Labrador. Boring time, but we were put up in retired air force barracks and fed bad food. The locals did what they could to make us comfortable.
Delta flew in a new plane, which broke too, but by then the mechanic brought in had fixed the original plane and the crew had enough rest they could take us on. No catering, so the pilot went to the local stores and brought out all the packaged sandwiches and candy he could find. My J class dinner was two cellophane-wrapped egg salad sandwiches. As the flight attendant handed my dessert of a Twix bar she said, "bet you wish you had taken the sundae."
Best part was on the way home. I wasn't upgraded but the same purser had my flight as on the flight that was disrupted. There were open rows in J, so after we took off, she brought me up for the entire flight, only sending me back to Y just before landing.
I had other complimentary upgrades TPAC, but this was my only one going to Europe. And it has the best story.