"A lot of stuff can come up in this pose, so don't be surprised if you start feeling sad (or nauseated or angry or bewildered or [add emotion here...]"
I've been to lots of yoga classes where the teacher says this at some point during the class. Normally, it's right when we're about to do Camel. But since I've never experienced any sort of Come-to-Jesus moment like that in the middle of practicing, I usually make some snarky comment to myself and just keep going.
Then today, as I was lying in savasana (corpse pose) at the end of Hatha class, I started to cry. Not because some old family trauma surfaced or even because I've had a particularly bad day. It was because my Best Mom Friend (BMF) is moving away next week, and I suddenly glimpsed how yucky life is going to be without her around.