The Oktoberfest. Or generally thinking that traveling to Bavaria means having been to Germany. It's like going to Texas.
Anything Bavarian outside of Bavaria. Seriously the Hofbräuhaus in Berlin is a tourist trap. Sure the beer is good, but it's hard to get sh*tty beer in Germany anyway. Skip the Hofbräuhaus in Munich as well and travel to one of the smaller cities in Bavaria. From experience I'd suggest going to the Allgäu. There's a brewery in every village on average. I'm sure Bavarians will chime in.
Avoid Munich is what I'm trying to say (though it does have great museums).
If you must go to Bavaria, be aware that Schloß Neuschwanstein is not an old castle but from the 19th century. It is stupid beautiful though and I wouldn't fault anyone from going there.
Something closer to home: large flea markets in Berlin, especially the one at the Mauerpark. It's a hipster, expat, and tourist trap. Oh and most restaurants close to famous sights are also overpriced.