According to German election laws, voters are not allowed to reveal what they voted for before leaving the polling station, writes German Bild and the news agency Reuters, among others.
Therefore, Armin Laschet's ballot paper quickly became a snack on social media.
But the election commissioner says that Laschet's vote still counts, because he - as expected - voted for his own party.
Reuters has sought a spokesperson for the CDU candidate, who did not respond.