I think one of the BIFs that I have tried-and-failed with the most must be partridge. It is mostly because their flight path is close to the ground, and the background is almost invariably some sort of green/brownish field - and that keeps baffling my AF. I hope when that video is released tomorrow, it's all going to be a lot of details about how their new-and-improved AF system provides accurate focus in situations when birds are flying past a background of earthy colours. Actually, I have a STRONG feeling that this will be the main message of that video tomorrow!
But meanwhile I actually DID manage a decent partridge for once!