I have come to realize that Field guides often point out "diagnostic" field marks that in fact really aren't. They may be the norm for a certain species, but not diagnostic by any stretch.
I have collected a sample of photos that demonstrate how a field guide can lead you down a bumpy ID path.
European Golden Plover
Almost.. but not quite
This plover shows an erect stance, pot-bellied breast, skinny "necktie", ect. Compare the structure to the Black-bellied plover behind it. It's an obvious difference. But, the ID didn't pass the muster
Little Stint
Almost..but not quite.
This bird shows rufous tones, a gape notch that doesn't really matter, and a split-supercillium among other traits. It was called a Semipalamated sandpiper.
Long-toed Stint
Almost..but not quite
This bird shows what every field guide says is diagnostic- a pale base to the lower mandible, dark centered scapulars, and an erect stance. Come to find out Least can also sport these traits. Who knew.
Common Ringed Plover
Almost..but not quite
This is a July bird in tattered plumage. No doubt a female, but no takers on the ringed Plover.
Aphrodite Fritllary
Almost..but not quite
Even butterfly guides stumble. The marks on the basal cells, as show w/ arrows, are noted in all guides as diagnostic for Aphrodite, but now always. This bug has been called a Great-spangled fritillary although the guides show clean basal cells for that species.
Hickory Hairstreak
Almost..but not quite
The guide states the blue HW cell needs to be longer than the adjacent orange cell which in this photo is almost 2x as long, but it's a Banded hairstreak.The moral of the story is field experience and hours of observation can usually trump the Field Guides