Both of these are correct, but does one have an emphasis that the other doesnt? Is one more powerful than another in the context of advice? How do the following two sentences differ?
A) It takes only one mistake to negate all of the good you have ever done.
and
B) It only takes one mistake to negate all of the good you have ever done.
A has greater emphasis on quantifying the advice. No matter the acceptable number of mistakes, one is the key to consequence.
B has greater emphasis on qualifying. So despite the acceptable number of mistakes, the warning to understand here is the first one carries potential for consequence.
Both sentences are about the fragility of your reputation, no matter how much good you do. They have the same meaning and power, but create their meaning in slightly different ways.
In sentence A, "only" modifies "one", so your reputation is fragile because of the low number of mistakes it takes to ruin it.
In sentence B, "only" modifies "takes one mistake", so your reputation is fragile because merely making a mistake can ruin it.
