Can Mistfall trigger by targeting an already buffed unit with a buff effect?

Ruling: No, Mistfall cannot trigger by targeting an already buffed unit with a buff effect. You must actually place a buff counter on a unit (i.e., the unit must not already have a buff) for Mistfall to trigger.

Nuances:

• Units with buff counters can still be chosen as targets for buff actions, but they will not be buffed as part of the execution
• The trigger requires that buffing actually occurs (a buff counter is placed), not just that a "buff a unit" effect resolves
• This is analogous to discard effects: if you have no cards to discard, "when you discard" triggers don't activate because you didn't actually perform the action