I've heard Tango, and in 2007 Tango, Juliet, and Kilo were popular: http://www.flyertalk.com/forum/unite...threads-3.html

To domer: If the first segment is late, the second segment will leave without the first one -- same situation as if they had different flight numbers.

I have seen UA sometimes hold a flight to accommodate misconnecting passengers from a late inbound, but the only thing special about the "same flight number" situation is that there may be a lot of misconnecting passengers if a lot of people bought the direct flight A-B-C on one flight number.