I may not really say such achievement is an accidental because the bettor has long been chasing the success even while he had been loosing on the long run.
So invariably such bettor is been an expectant that to earn profits is the goal to why he is being a gambler and as well worth taking the risks of staking and loosing their values.

Instead of to say it is an accidental occurance, it is better to give it the normal term of winning on gambling is being based on "LUCK". 

Even in the lottery, the successful persons can not be said to had won accidentally because they on their own intentionally purchases the ticket (s) and hope that they will be the lucky ones after the system is shuffled to pick the winner.

So then, the winner is said to had won by luck while being hopeful to win. So since the processes did not happened like a "manna from heaven", it is not an accidental success but lucky.