Real Madrid's (temporary) round of 16 curse
Real Madrid are the clear dominators of the European Cup or UEFA Champions League, that leaves no room for doubt, however, there was a period in which they were unable to get past the round of 16 and in fact, they were eliminated at that stage for 6 consecutive seasons. The inability of the white club to get past that stage was even called a curse, although, as you can see, there were several things that led to that situation. Clearly, the stability in all senses that the club had with Ancelotti or Zidane in charge, was not there in that period and this inevitably affected a competition as difficult as the Champions League.
A bit of context
In the 2003-04 season, Real Madrid were in the midst of the Galactica era, Beckham had just arrived at a team that already had Luis Figo, Zinedine Zidane and Ronaldo. In addition, in the previous three even-numbered years, the white club had won the UEFA Champions League (1998, 2000 and 2002), something that seemed within reach that season with the great accumulation of talent.However, the team had lost a lot of balance with the departure of Makelele to Chelsea, had lost depth in the squad due to the departures of players such as Fernando Hierro, Steve McManaman and Fernando Morientes, as well as losing manager Vicente Del Bosque.
That season, Real Madrid reached the quarter-finals of the Champions League against Monaco, where Morientes was on loan. The first leg was won by the Madrid team by 4-2, however, in the return leg, the French team won by 3-1 with a double from Ludovic Giuly and a goal from Morientes, eliminating Real Madrid on the away goal rule, still in force at that time.
The beginning of the curse
The following season, the white team sought to strengthen itself better, especially in defence, signing the defenders Walter Samuel and Jonathan Woodgate (who would not make his debut with the club that season) and in winter, Thomas Gravesen, in that search for a replacement for Makelele, in addition to that year's Galactico, Michael Owen. On the bench, Anotonio Camacho resigned as manager at the beginning of the season and was replaced by Mariano Garcia Remon, until the arrival of Vanderlei Luxemburgo at the end of December 2004.The first half of that season, the team had been very inconsistent, to the point that they qualified for the round of 16 as second in their group, just one point ahead of third-placed Dynamo Kyiv. Luxemburgo gave the team some stability at the beginning of 2005, but the rival that awaited them was not easy, Juventus, with Capello as manager and Zlatan Ibrahimovic and Alessandro Del Piero with company on the pitch.
In the first leg, at the Santiago Bernabeu stadium, Real Madrid won 1-0 with a goal from Helguera, but in the second leg, it was Juventus, with a goal from David Trezeguet, who would take the lead. At the end of the 90 minutes, the aggregate score was 1-1 and it went to extra time where in the 116th minute, the Uruguayan Marcelo Zalayeta, would score the goal that qualified his team and unwittingly began Real Madrid's curse in the round of 16 of the competition.
In the summer of 2005, Florentino Perez changed his project a bit and brought in players who perhaps did not fit the profile of a Galactico and let go some stars like for example Figo. The club was not very stable as a club in genrela, and this was reflected in the transfer market as well as in other decisions. Luxemburgo would be dismissed from his post almost a year after his arrival and his place would be taken by Juan Ramon Lopez Caro. In that edition of the Champions League, Real Madrid would again qualify second in their group, although with a little more ease.
Their opponent in the round of 16 was Wenger's Arsenal, who still had a certain base of The Invincibles. In the first leg, the Gunners would win 1-0 at the Santiago Bernabeu with a fantastic goal from Henry who left several players behind to subtly define against Casillas. That goal would be the only one in the entire tie and would be enough for the Madrid team to be eliminated from the tournament. Arsenal eventually reached the final, which they lost to Barcelona. In the middle of the tie, Florentino Perez resigned as president of Real Madrid.

Fabio Capello came in as Real Madrid coach
The curse goes on and on
A new era began at Real Madrid, with Ramón Calderón as president, Pedja Mijatovic as director of football, Fabio Capello as manager and a renewed squad where names like Ronaldo (in winter) or Zidane (retired) were released and players trusted by the Italian coach such as Cannavaro or Emerson arrived, as well as some youngsters like Gonzalo Higuaín and Marcelo in the winter transfer market. Once again, the team qualified second in its group (something that affected the round of 16 match) and had to face Bayern Munich who, at that time, was seeing the return of Ottmar Hitzfeld to the Bavarian bench. Although Real Madrid won 3-2 in the first leg played in Spain, the Munich team won 2-1 in Germany and once again the away goal rule was detrimental to the Madrid team.For the 2007-08 season, Calderon had turned the squad around, leaving the Galacticos' core behind and forming a more balanced squad. Capello had left and now the German Bernd Schuster was the manager of a squad that included signings such as Pepe, Arjen Robben and Wesley Sneijder. On this occasion, Real Madrid seemed to be more solid both sportingly and institutionally and this was evident in the pitch, dominating the Spanish league throughout the year and qualifying first in their Champions League group (by goals scored compared to second). In the round of 16, Luciano Spalletti's Roma awaited them, with Totti in one of his best moments, playing as a false nine. The Italian team beat the Madrid team 2-1 in both the first and second legs, leaving them once again in the round of 16.
Real Madrid would once again find themselves in a difficult institutional situation, as in 2008-09, Ramon Calderon would leave the club's presidency, while there were also changes in the pitch, as Schuster was replaced by Juande Ramos in the middle of the season. A second place finish in the group stage would lead the Madrid club to face Gerrard and Torres' Liverpool in the round of 16. In this case, the English team would easily beat Real Madrid by an aggregate score of 5-0, beating them 4-0 at Anfield. The following season would see the return of Florentino Perez to the presidency and the Galacticos 2.0, with the arrival of Cristiano Ronaldo, Kaká, Karim Benzema and Xabi Alonso as well as Manuel Pellegrini as manager.
The reality is that everything looked like the curse would end that year. The team finished first in its group and was drawn against Lyon in the round of 16, a club that had already given Real Madrid a hard time in previous years and this would be no exception. After a 1-0 defeat by the Spanish team in France, everything was decided in Spain, where Cristiano Ronaldo quickly equalised the aggregate score, however, a goal from Miralem Pjanic in the 75th minute sentenced the white club, once again.
The following year, with the arrival of Mourinho to the Whites' bench, the curse would be broken as the team would reach the semi-finals (losing to Barcelona), having beaten Lyon in the round of 16 and Tottenham in the quarterfinals.