I and two family members traveling together were once upgraded from "last class" to first class (in a 747) flying from Seattle to London. This was 1989, so YMMV.
We arrived at the Seattle departure desk about 40 minutes before the cut-off time 45(?) minutes before departure, but had to join a long queue. At right about the cut-off time, we got to the desk. The agent told me the flight is full. I glanced at the clock and said "Well, we're here on time; what are you going to do about it?" She seemed none-too-pleased, but clacked away at the keyboard for a minute or two. I was extremely curious what she was doing, but uncharacteristically remained quiet. She then provided three new tickets (which was how boarding passes were done then), and said "I'm upgrading you to first class. Have a nice flight."
I had negotiated aggressively months before for excellent fares; likely we paid among the least of anyone on that flight, approximately $375 (round trip) per person for a late August two week trip when the going rate was $675–750. My mom was seated next to a highly opinionated London businessman. When they got to discussing ticket prices he asked for proof and then harangued a flight attendant mercilessly for charging him $3,500 for a one way ticket and being seated next to someone paying less than a tenth of what he did.
I expected that for the low fare we paid, we'd still get coach food service, but no: we got all the goodies (travel kit, complementary towel, etc.), food, and drink—like free cocktails and wine—just like all the other first class passengers.
No such luck on the return flight, and it hasn't happened to me since even after traveling for more than four million miles.