These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Nor did he write "In the Hall of the Mountain King."
Music to Invade Poland to: Wagner is often used as background music for scenes of war-related activities (including World War II). This is partly because Those Wacky Nazis actually were fond of Wagner's music and occasionally used it in their propaganda, which has given it Unfortunate Implications in some circles to this day. Since Wagner himself died long before the Nazis rose to power, this may be largely a case of Hitler Ate Sugar; however, it's complicated by the fact that he is also on record with some nasty anti-semitic statements of his own. On the other hand, complicating things still further are the fact that Wagner was also a left-leaning socialist for much of his life, befriended Russian anarchist Mikhail Bakhunin and participated in the Dresden May Uprising (which got him exiled by the Saxon government), and expressed a strong distrust of power in his works (along with a belief in The Power of Love). In other words, much like the case of his erstwhile friend Friedrich Nietzsche, it is quite likely that his untimely death resulted in his appropriation by people of whom he is unlikely to have approved in the slightest.
Older Than They Think: Many people think that the saxophones were invented in the Jazz Age, but Wagner had requested Adolphe Sax to figure out how to create an instrument to play a smooth brass/woodwind sound back in 1840.
Seinfeld Is Unfunny: Wagner's „Musik der Zukunft‟ ("The Music of the Future") was considered daringly, even outrageously, innovative in his own time; but he became so influential that his music is now reckoned old-fashioned and even stereotypical by some.
Wagner's leitmotif technique - that is, associating one musical idea with a particular character, item or feeling and repeating it whenever that/they recurred - was revolutionary at the time, but is standard practice in film music today. His writings also had a huge influence on the development of musical theater.