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Obviously what Leverkusen did last season happens once a decade for a team of this level, if not less often. Bayern are looking very confident at the moment, only 6 games in and they share first place with Leipzig on points but are ahead of them on goals, but I'm sure they have a better chance than Leipzig on the long run, even though Bayern have to spend their energy on the Champions League too
Let's agree on less often!
Or maybe you are not wrong as there is a list on wikipedia. It does happen once in a while in the less prolific leagues in Europe, but in Germany it was the first time ever, unless we want to count Dresden in 1943 during the Nazi times. In England Arsenal did it in 2004 and guess what, before that it was Preston North End in 1889!
In Italy it actually happened 5 times in 110 years. In Spain it is almost 100 years ago when Real Madrid did it.
I am uncertain about Bayern right now as they have similar issues all the time. The ball possession is almost never a problem, but once they have got the ball, they don't know how to overcome the defense and they concede goals from teams that barely have the ball. Aston Villa scored a late one, not to mention Frankfurt. They score so many goals against weak opponents in games that are not unimportant, but not the real deal either. Aston Villa was a slap in the face and now against Barcelona the next slap could follow.
Leverkusen had two games that went all wrong. The game against Leipzig, which Leipzig won 3-2 away from home and then giving away the 2-0 lead against Kiel. But I think they have a very realistic chance to defend the title.