I always liked this quote, for when I got ahead of myself:
"No battle plan survives contact with the enemy."
for which BrainyQuote credits Colin Powell, but it's not quite accurate, that quote was actually paraphrased by Correlli Barnett in the The Swordbearers: Studies in Supreme Command in the First World War (1963), but that, like I said, was paraphrased. The original quote is
"No operation extends with any certainty beyond the first encounter with the main body of the enemy." ,
which was said by Helmuth von Moltke the Elder (January 1871 – August 1888), who was a massive admirer of Napoleon and made Napoleon's saying
"I have never had a plan of operations."