Actually, most of Middlesex appears ok - but the registration districts of Barnet and Edmonton (at least) have been indexed under Berkshire.
This seems to be because for 1861, these two RDs have high out-of-sequence piece numbers - in high 700's, while most of Middlesex runs from 000's to 100's. Other 700's are in Berkshire.
I have informed Ancestry and pointed out what the problem is (that they don't seem to have allowed for out-of-sequence piece numbers) - and that the problem probably affects various RDs in other counties. However (strangely) they <i>have</i> got all those notorious "Welsh" pieces in Herefordshire right!
Apart from this glitch, I'm really surprised at how good the indexing is in general! Have found lots of AWOL ancestors already... using "less is more" principle when searching...
Steve