Keith Vinicombe has posted a shot on Stop Press of an interesting Goldeneye at Chew that has been difficult to pin down apparently. It's being labelled as Presumed Barrow's x Common Goldeneye hybrid (ad female). Comments from Keith are: "Head shape, head colour, size and culmen profile suggest Barrow's but upperwing pattern, lack of strong bulge at rear of head, extent of brown on fore-neck and bill length suggest Common." Here's a direct link to photo for future reference.
There are other photos on Surfbirds of hybrids. Here's a hybrid male Barrow's x Common Goldeneye by Bruce Finnan.
For reference, here are some straightforward Barrow's:
Nice head shape comparison from Bill Schmoker. Barrow's female is 2nd from right
Close up of a female head shape by Daniel Bergman
Birding magazine published something on hybrid goldeneyes back in mid 90s. I'll have to look to see if I have it and can scan a copy if anyone is interested. I feel there are too many open and unsolved ID issues hanging out on this forum