This Bible contradiction is from the Skeptic's Annotated Bible.
The Philistine garrison in Geba
1 Samuel 13:3 View context
And Jonathan smote the garrison of the Philistines that [was] in Geba, and the Philistines heard [of it]. And Saul blew the trumpet throughout all the land, saying, Let the Hebrews hear.
After the Geba smiting
1 Samuel 14:14 View context
And that first slaughter, which Jonathan and his armourbearer made, was about twenty men, within as it were an half acre of land, [which] a yoke [of oxen might plow].