Originally Posted by
IggySD
Reusing it from what? I looked on flightaware and that flight goes SEA-SFO-SAN every day and is just 431 for both legs. That day (15Jul) it was specifically listed as 431 Tango. I was going to ask the pilot but the beers I had before leaving SFO caught up with me as soon as the fasten seatbelt sign came on for landing so was in a bit of a hurry.
Some flight numbers are reused intermittently. For example, UA431 will be used for an IAH-DEN leg starting 9/5/07. Maybe there was another UA431 on the day you flew; I don't know of any easy way to go back and check.