Someone who knows more about anatomy than anyone I've ever met once said to me: "Half of all yoga poses are great if done correctly, and half of them should never be done by anyone." He mentioned the pigeon pose as a particular instance of a pose that may feel like it's "opening you up" but which, if you're doing it perfectly, is stretching something (hip ligaments) that don't need to - and shouldn't - be stretched.

EDIT: Downvoter, if you know otherwise, please educate me!