If your opponent fouls and leaves you snookered on all 15 reds, you get a free ball. This means you can play any colour, which if you pot it counts as a red. You can then play another colour; if you play the black, and pot it, you now have eight points. If you then pot all 15 reds, a black with each red, and finally all six colours, you'll have scored a total of 155 points.
Alex Higgins was said to have made a 155 break – but "not ... in a regular competition", meaning that it's "not officially recorded".
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