own goal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A goal scored by a defender on his or her team's goal by a mistaken play, such as a misdirected pass.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun sports A goal that results from a player putting the ball or puck into the goal of his or her own team; the resulting goal being scored for the opposition.
• noun figuratively A blunder that damages one's own prospects.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (soccer) a goal that results when a player inadvertently knocks the ball into the goal he is defending