Surprised to see a reddit like question being asked here on hackernews.

Water, soap, oil, compressed air etc will not work. Ask your friend to gently tap around the edges of the container that is stuck with either a wooden spatula or the back of a wooden ladle. Gentle taps in the direction of getting unstuck all around the edge of the stuck container. May require few minutes of tapping.

Source: This has happened to me many times.