YMMV / Georg Philipp Telemann

• It Was Popular, So It Sucks: Telemann was the most (commercially) successful and highly regarded German composer of his time and in the course of a long and productive life wrote more music than Bach and Händel combined. After Bach was rediscovered in the mid-19th century, it became fashionable among musicologists and music-lovers to regard him as the great genius of Baroque Music. Since Bach had not been very successful commercially, it was easy to paint him as a misunderstood and unappreciated master who wrote for eternity while the "shallow" Telemann marketed himself by pandering to the tastes of the uncouth audience and producing notes as if in a factory. It would take over a century, during which other German Baroque composers besides Bach and Händel were rediscovered and reappreciated, for Telemann to (largely) regain his original stature. The ironic thing was that Bach and Telemann admired each other and Bach not only copied out by hand entire cantatas written by Telemann, but would also sometimes incorporate Telemann scores into his own works, as was perfectly normal in the 18th century (before copyright laws). And then admirers of Bach like Philipp Spitta and Albert Schweitzer (yes, that Albert Schweitzer) inadvertently used pieces attributed to Bach but actually written by Telemann (as it turned out after musicologists properly took stock of the surviving part of Telemann's huge oeuvre) to "prove" the nigh-infinite superiority of Bach over Telemann.
• Misattributed Music: Some works by Telemann (and other composers) were misattributed to Bach because the people who compiled the official Bach-Werke-Verzeichnis were not sufficiently familiar with the works of Bach's contemporaries. For instance, it eventually emerged that the cantata Ich weiß, daß mein Erlöser lebt (BWV 160) is completely by Georg Philipp Telemann, as is the opening chorale to Ich lebe, mein Herze, zu deinem Ergötzen (BWV 145).
• One-Hit Wonder: While Telemann is one of the most prolific composers, he's most likely mainly known in the modern day for his viola concerto. The piece is used as a popular teaching method for people learning viola and is very frequently performed and recorded.