This morning I was reviewing offers on Peerfly and I came across a "top converting offer" for an online game. In the details, however, they specify that the only allowed country is "Poland".
While I'm certainly not saying that they are doing anything "wrong" I do think they are gaming the system a bit by presenting an offer that looks amazing, but that only pays on traffic from one specific country. I have to assume that there are ALOT of affiliate marketers who miss that detail, either because of poor English skills or because of poor attention to detail, resulting in people spending money promoting offers which make the advertiser money, but which won't make the publisher a dime.
My question for the STM community is, how common is this?
Note: I used to make high four figures per month promoting Clickbank offers a few years ago, and now I'm returning to AM after a lengthy break, so I'm just getting my feet wet with Peerfly and other networks (since Clickbank is blocked as a destination on Facebook and some other networks these days).
Good observation, hope that some expert members answer your question
This is quite common and one of the first things that experienced affiliates do is read over the terms of the offer.
I would not say this is gaming the system. This is the system.
^^ THIS IS IT ^^
This is the system indeed, not gaming it 
Vortex gave you a detailed explanation already so I'm not gonna write more about it. Always read the terms, offers have more than GEO restricions - some don't accept certain traffic types, some want particular mobile carriers only - it's 100% your responsibility to follow these restrictions.