The FIFA Club World Cup winner's badges has not always brought good luck to the team that accomplishes the feat, as Carlo Ancelotti's Real Madrid from the 2014/15 season are the perfect reminders of.
More often than not, it has signalled that the apex has already been reached, making it more difficult to stay at the top. Here are the instances of teams who have struggled that particuar curse of the badge:

End to 22-match unbeaten run and no trophies by the end of the season
As mentioned, Los Blancos are better aware of the curse than most. Carlo Ancelotti's side had won the Copa del Rey, the Champions League, UEFA Super Cup and FIFA Club World Cup to end 2014 in great spirits, but the team was left in tatters by the time Rafa Benitez left it in December 2015.
Before Benitez's appointment, a 4-0 loss at the Estadio Vicente Calderon, a Champions League exit, throwing away a six point lead in LaLiga all meant that Ancelotti had to go, a shocking end to his tenure after the football on display just a few months before.
Benitez's June arrival was a poor fit from the start, as matters went from bad to worse.

Barcelona's lost hegemony in 2016
Zidane's side achieved more victories than Luis Enrique's in the calendar year and captured the most important trophy of all - the Champions League.
Barcelona in fact made a quarter-final exit to eventual runners up Atletico Madrid.
In two Clasicos that year, Real won one and drew the other despite both being held at the Camp Nou.
Guardiola's Bayern doesn't meet expectations
Inheriting a treble winning squad meant no matter what Pep Guardiola did, it'd be impossible to beat the season the club had just had.
Nonetheless, not becoming back-to-back winners of the Champions League left a tough mark on Guardiola's legacy.
The 5-0 defeat over two legs to Real Madrid rank amongst the worst of the Catalan coach's career.

Corinthians lose Libetadores and finish 10th in Brazilian league
It is not just European teams who are cursed, as back in 2012 Corinthians too felt the weight of the Club World Cup badge.
Despite beating Chelsea in the final, they couldn't repeat their Copa Libertadores success and ended the year in poor form.

Inter fails to win consecutive Scudetto and falls to Schalke in Champions League
After Jose Mourinho's historic Inter side won the treble in 2009/10, Rafa Benitez took over at San Siro.
Despite winning the Club World Cup, he did little to continue his predecessor's success.