All balls 'on' are potable in any stroke. That's why you can pot any number of reds when reds are 'on' and score them all, and why you can pot both the nominated (free) ball and the snookered ball 'on' in a stroke, as they are both 'on'. Potting any ball that is not 'on' in a stroke is always a foul. It's pretty simple.One of the key moments in that 16th frame was when O'Sullivan fouled by fluking a red when he potted an "on" blue. I take it this is an example of the foul called "causing a ball not on to be pocketed."
Does anyone here know the logic of applying this rule when the ball "on" is also pocketed? And bear in mind that this is a game in which flukes are usually counted and in which potting two balls on the same stroke is legal in certain other circumstances.