If you can spot the error please do not post the explanation let other people have fun solving it. For the first few days. If you do and I see it I will delete it.

Notice that $\displaystyle x^2=x+x+x...+x$ where $\displaystyle x$ appears $\displaystyle x$ times. Thus, taking the derivative of both sides,

$\displaystyle 2x=1+1+1...+1$ but 1 appears $\displaystyle x$ times thus, $\displaystyle 2x=x$? What is wrong?