Well worst mistake would be to invest too much time and resource to gambling. Let's face it gambling is a game of chance, there's no guarantee in gambling so making a bet that would either make or break you financially and emotionally would be a very big mistake
i personally did mistakes for so many time and for each time is that i lost the money beyond my limit, the worst mistake i did in gambling in cricket match,the match was between Pakistan and India, the India position was very clear and it was 99 percent winning game for India but i chose Pakistan for winning although the lost was not too much but still i boot if in advance.