I've been seeing a lot of misidentifications - or perhaps wishful ones - of the invasive Chelidonium majus, greater celandine as the Eastern U.S. native Stylophorum diphyllum, celandine poppy. Here is a visual guide for distinguishing them.
HabitBoth grow to similar height and width, holding their flowers just above the foliage when blooming.
All parts of both species exude a brightly colored sap when broken or crushed. However, I find the color is variable, ranging yellow to orange, and not distinct enough to be diagnostic.
FoliageBoth have deeply pinnate leaves with lobed leaflets. The lobes on the native celandine poppy are more open, almost oak-like, than on the invasive greater celandine.
Fruit (Seedpods)I find this the easiest way to distinguish the two species.