The recent thread about a Pied Bushchat being dismissed in the past as a potential vagrant makes me wonder what else this might apply to?
One obvious one I can think of: Sulphur-bellied Warbler Phylloscopus griseolus. Breeding, and wintering, ranges very similar to Hume's Warbler Phylloscopus humei. Numerous UK records of Hume's and presumably plenty more elsewhere in Europe. No European records of Sulphur-bellied, as far as I'm aware. Are they being overlooked as 'rather bright Chiffchaffs'?
Anyone want to suggest others?