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)