Two of my favorite epigrams:

• In theory, there is no difference between theory and practice, but in practice, there is. -- Jan L. A. van de Snepscheut
• The string is a stark data structure and everywhere it is passed there is much duplication of process. It is a perfect vehicle for hiding information. -- Alan J. Perlis

What about yours?

[Updated to add:]

‘Tis not in mortals to command success,
But we’ll do more, Sempronius – we’ll deserve it.
-- Joseph Addison’s Cato, a Tragedy (1712)