Does Mistfall trigger when you attempt to buff a unit that already has a buff counter?

Ruling: Mistfall does not trigger when you attempt to buff a unit that already has a buff counter. You are only considered to be "buffing" a unit if you effectively put a buff token on it.

Nuances:

• Choosing a unit to buff and actually buffing it are two different things
• If Mistfall said "When you choose a friendly unit to buff" it would work differently, but it requires an actual buff to occur
• The trigger "When you buff a unit" can be understood as "When a buff token is put on a unit"
• Lee Sin Green is an exception to the normal buff counter rules