Try Harder

The essence of Dirac's problem was simply that: $\sqrt{p^2c^2 + m_0^2c^4} \neq pc + m_0c^2$ and, in general: $\sqrt{a^2 + b^2} \neq a + b$ (for non-zero $a$ and $b$), as everyone learns (or should learn) at school.

At which point mere mortals give up. Geniuses try harder.

(Norman McCubbin, "Beauty in physics: the legacy of Paul Dirac" , in Contemporary Physics, Volume 45, Number 4, July-August 2004, pp. 319–333)