I haven't had this particular combo in a random game, but I realized it would have an interesting effect: You could empty out the entire Berserker pile in a single turn
Play a Berserker. Choose Trail for the cheaper card. Use Trail's reaction to play it, but play it with Way of the Butterfly. Return it to the Supply for another Berserker. Play that one. Keep repeating until the Berserker pile is empty. An interesting quirk here is that since all of this is reacting to the very first Berserker's play, the discard doesn't happen until the Berserker pile is empty, which could theoretically be relevant if your opponent has a Monkey in play. You could also start this process by playing a Trail from your hand if you have at least one other Action card already in play. A similar trick would work with Villa. In a Shelters game, this could even happen on turn 1 (although it wouldn't really be a sensible move to fill your deck up with 10 Berserkers so early) - play your Necropolis and 4 Coppers, buy a Trail, play it with Butterfly for Berserker, and then you've got this process started
Replacing Trail with Border Village allows instant emptying of the BV pile. With at least one other Action card in play, you play a Berserker from your hand as Butterfly. Return it to its pile to gain a BV, then choose Berserker as the cheaper card (thus getting back the same Berserker you'd just returned) and repeat the process until the BV pile is empty, or, alternately, you buy a BV, gain a Berserker, play it with Butterfly, and start the cycle from there
With both Trail and BV, you could thus empty two piles in a single turn. Potentially before your first shuffle in a Shelters game with the right shuffle luck. And then, with a deck full of Border Villages and Berserkers, you could gain all the Duchies in a few turns (gain a Trail, play it with Butterfly for a Duchy, using the BV's to play multiple Berserkers each turn). I tested that out in a bot game and ended the game on turn 6 with all 8 Duchies, and the BV and Berserker piles empty, plus one VP token from a Chariot Race, for a ending score of 25-0
Obviously not a combo that's going to come up very much, so not exactly a practical strategy, but pretty wild to play