own goal

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. 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.
• n. A blunder that damages one's own prospects.

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

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