Happened to me a few weeks ago, flew to Eindhoven instead of Charleroi.
Morning 6am flight, both Wizzair from Budapest (external boarding gates with variable fences) next to each other. Was almost last pax, ran to the plane, apparently the wrong one. Flight was half empty, I realized after doors closed. As my destination was Antwerp (halfway between the two airports) I was like lets see whats happen
Nothing happened (both EU airports with no border or passport control), but got a good story.
Wizzair cabin crew since a few months DO NOT check passes when boarding. Mistake in my oppinion but everything for the turnover time.