What comes once in a minute, twice in a moment, but never in a thousand years?
The letter M.
Why this works
This riddle plays with the concept of frequency in language rather than time. The letter "M" appears once in the word "minute," twice in the word "moment," and not at all in the phrase "a thousand years." The cleverness lies in shifting the focus from a temporal measurement to the letters themselves, making us think outside the box!