He can't yet roll, crawl or sit. I deposited him in the corner of the couch (tucked in between pillows) and ran to the loo. Heard an enormous thud, followed by a milk-curdling scream - he'd fallen off, and was now lying on the wooden floor on his back. He stopped crying after a few minutes (I didn't) and played as before, but got antsy about an hour later, so I brought bedtime forward a little and tucked him in. He's been asleep since. I can't see or feel a lump, bruise or external injury. And I still don't know how he did it - the couch is huge.
How likely is it that he's hurt himself badly? What should I look out for?