United actually did this on one of their first flights of their 777 (back when I worked for them). Took off smooth on a MIA-ORD flight, got to altitude, then the pilot came on and told us he is considering whether he wants to turn on the second engine...

You should see people's looks when they find out only one engine was on...

(ok, no actual passengers were on this test flight, but even employees get scared sometimes)