Same in the UK (as Mysticete probably knows already): issues surrounding whether introduced species are self-sustaining (and hence Category C in British terms) and on ship assistance are continual areas of debate. Very few (myself included, I admit) have taken Lady Amherst's Pheasant off their lists, despite it having been on Cat C but died out since. As for Nearctic vagrants, single records of Brown Thrasher and Eastern Towhee in the 1960s remain on the BOU list despite many feeling they could not have made it across without ship assistance, while two Nearctic sparrows I have seen (Song and White-crowned) were on a nature reserve within Liverpool docks, though both have occurred multiple times before and since. A few House Finch records have not made the grade though. As an extreme, BOU did understandably draw the line at the Snowy Sheathbill which came back from the Falklands on board a Navy vessel and was known to have been repeatedly fed by ship's personnel on the trip back.