So this Barred Antshrike record has gotten us thinking. Yes, dangerous, we know.
Assuming the record is accepted by the ABA, then Martin Hagne is the only birder (well, his wife heard it, too, but we'll assume she is not a birder) who will have this species on their list for the ABA area.
Are there any other species that have only occurred once in the ABA area, have been accepted by the relevant state records committee and eventually added to the list by the ABA, but have only been seen by one person, by the initial observer? Since "firsts" these days usually require hard documentation (ie photo, sound recording, or specimen), you'd have to see the bird, record it, and then no one else would ever see that individual bird (or any other bird of the same species) after the original sighting.
What about in the U.K.?