In its federal election, Germany uses mixed-member proportional representation, where a voter makes two votes: one picks a particular candidate in local constituency, second for a party. In short, a constituency seat is won by candidate who reached plurality of votes. Parties, that have achieved 5% votes on party lists are allocated extra seats so that at the end of the day percentage of their seats roughly corresponds percentage of votes they had received.
As a result, major parties get most of their seats from constituencies, while smaller non-regionalist parties rely on party lists. For example, in last Sunday's election CDU won 185 constituencies and 15 party list seats, with FDP 0 and 80, respectively.
Therefore, a major party might be tempted to have no candidates run in the first list and endorse independent candidates that are close to their ideology. Assuming most voters know who is endorsed by his/her favourite party, independents in question will win their races and the major party will still win a bunch of "compensation" seats, since it didn't technically win any constituency race.
Taking recent election as an example, we could have 185 CDU-leaning and -endorsed independents, but CDU would also win an insane number of party list seats (about 200, not sure about precise algorithm calculating "compensations"). That would easily give them majority.
Is there any system in place to avoid such shenanigans?