The violin is a lot more prestigeful and has more difficult parts than the viola, so violinists kind of see them as those who wanted to play the violin but weren't good enough. I guess it actually has to do more with the fact that nobody ever notices the viola since they are awkwardly in the middle, and that they therefore have less motivation to bother to count bars or play correctly in general. Either way they do tend to either forget to play or play the wrong notes every once in a while.

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