I have had three aborted landing experiences, but none on UA. The first was TWA coming into LAX and that time we were told (IIRC) that we were coming in to high so we were risking not having enough runway. My second time was on CO coming into Cleveland where there was another plane on the runway and the third was on LH coming into JFK where we were pretty much almost on the ground before going right back up and then circling at 2000 feet for about 20-30 min before landing. A woman across the aisle from me then started throwing up violently and the guy in the seat next to me starting panicking. Fortunately I was able to talk him down and get him to relax, but the projectile vomiting was not very pleasant to witness. No explanation was given for the aborted landing--only an announcement that we were going to circle before coming back to land.