In my brief, but glorious career as a twitcher, I saw a number of highly desirable species that had become blockers. For those who do not know what a blocker is, it is a bird whose rarity has not been diminished by turning up again enabling other birders to see it.
I used to have a good list of blockers. They are slowly dwindling in number...
The Semipalmated Plover in Devon was an UNBLOCKER
The Belted Kingfisher of a few Aprils ago was an UNBLOCKER
And now, the cherished Orphean Warbler that I watched on St. Marys in 1981 has been UNBLOCKED.
Just keep away from my Wallcreepers, Varied Thrush, Hudsonian Godwit, Little Whimbrel, Golden-winged Warbler... I need them as currency for bragging rights!