A game of two halves

Meaning

Circumstances have changed suddenly.

Origin

This football (soccer) commentator's cliche refers to games which have a different character in the two halves. The source of the phrase isn't clear. It was in wide use by 1990 when it was listed in a posting in the Usenet newsgroup - rec.sport.soccer. That posting cited a 'soccer dictionary' of 1988/89 as the source.