Can Nocturne be played directly to a defending battlefield after seeing it from Teemo's trigger on flip or when defending?

Ruling: Yes, Nocturne can be played directly to a defending battlefield after being revealed by Teemo's trigger.

Nuances:

• Teemo's trigger was errata'd to only trigger when defending, not when played or flipped from hidden
• When Teemo is flipped from hidden while defending, it becomes a defender and the "when I defend" trigger activates
• "When I attack/defend" triggers happen the first time each unit gets designated as attacker/defender in combat, regardless of when during that combat they first got designated
• Units that arrive late to combat (by being played like Shen, Teemo, Nocturne, or by moving in like Ride the Wind) get those triggers when they arrive