Little Blue Heron Egretta caerulea
The first record for Europe (not including the Azores) was an immature bird that arrived in Ireland in October 2008. Later the same autumn another bird was reported in south Wales but despite many hundreds of hopeful birders scouring the estuary the following day it could not be re-found - unless, as I suspect, it was a misidentified Little Egret. Young birds are white like Little Egrets and there was one Little Egret present the next day which had green legs rather like those of immature Little Blue Heron.
In North America it is mainly confined to coastal states in the south and east - with just a few in California, mainly in the San Diego area.