How can a completed match become a walkover?

Question

During yesterday 2018 World Cup's match between Panama and Tunisia, a Polish-speaking sport commentator said that one of these countries (didn't hear exactly which one) had a football match with Cape Verde team which they lost and which was later ruled out as a walkover.

How can an already played match, lost by one of teams, later be declared walkover?

So far (up until yesterday) I was more than sure that a walkover can only happen when -- as name suggests -- one of the teams don't even show up to play the game.

Side question: About exactly which match Polish commentator was talking about?

Answer 1

Here's what Wikipedia article on the term walkover suggests:

A walkover is the awarding of a victory to a contestant because there are no other contestants or the other contestants have been disqualified or have forfeited.

So,

How can an already played match, lost by one of teams, later be declared walkover?

Easy, if it's later identified that a team fielded an ineligible player, or due to crowd violence, etc.

Side question: About exactly which match Polish commentator was talking about?

As @Martin pointed out in the comments, most probably the commentator was referring to the incident of Fernando Varela playing for Cape Verde against Tunisia during his suspension.