I'm a major Apple-Homer, but I've had the following funny 2 SIRI experiences:
When told to get 6 cups of Half-N-Half at the store, and it was sold in Quarts, I asked Siri how many cups are in a Quart (4). Then how many cups are in a Pint (2). Two cartons. Done.
Worked perfectly. It was my 1st "real" use of Siri besides playing.
Another time I was driving past a lot for sale, 450ft by 650ft or something like that. I asked Siri, "what is 450ft times 650ft". Wolphram Alpha gave every conceivable area measurement (including the 6.5 acres I was looking for).
In both cases, when I try to repeat the questions in front of other people to show how cool Siri is, THAT's when it bombs. I tried the cups-things 3 times or more for others, and the acre-age one once.
So the real message is that Siri is a little shy, and a little bit jealous of you pimping out her skills to other people (esp my Android friends).