A מיגו is a logical argument used multiple times throughout the Rishonim and Achronim (for example, the Sha"Ch has a very long discussion about it here: https://www.sefaria.org.il/Siftei_Kohen_on_Shulchan_Arukh%2C_Choshen_Mishpat%2C_Dinei_Migo?lang=he)
The way the argument works is basically like this: if a claimant claims one thing, and he could have claimed something else that would have been advantageous to him, without any downside, then I believe him, seeing as if he had wanted to lie he could have lied in a 'better' way.
One example of a Migo is in Bava Basra (32b) in which a man goes to Rabbah Bar Nachmani with a bill. When the defendant claims it's forged, the man agrees, but claims he had a real bill beforehand and it had gotten lost. Rabbah Bar Nachmani said the man gets the money, Migo that he could have lied and said "it's not forged".
Seeing as some specific examples of Migos are readily available, and in theory a cheater or thief could learn them and use one to his advantage and claim the worse claim in order to remove any suspicion from himself, would we still use Migos today?
For example, in the gemara mentioned above, if the man had knowledge of Migos, we might worry that he actually had never had a bill, and now he's lying by saying that it was forged but he had a real one that got lost (knowing he'll be believed because of Migo). And he won't claim the 'better' lie, that it's a real bill, for any number of secondary reasons, which in classical Migo-rules wouldn't cancel the Migo (maybe he's worried that at some point in the future the bill will be revealed as a forgery, and he'd like to circumvent that, for example).
I've seen this potential problem discussed, I don't remember where. Does anybody know where this can be found, and what the mefarshim answer?
I don't know how to tag this, suggestions/ edits are appreciated.