Somewhere, most likely Florida, there is an endlessly flowing fountain of crocodile tears called the Fountain of Doubt. Men sit by the Fountain, eager to con you, so they tell you to drink. If you don't give them what they want, they tell you that you had promised to drink from the fountain and you haven't yet. Drink. But you did drink, so you're filled with doubt and you doubt your memory of drinking.
The thing to remember about the benefit of the doubt is that without being inside your head, no one can know that you haven't given it already; if they say you haven't, that's what they've assumed. This assumption is often self-serving, with people saying that you haven't given them the benefit of the doubt because you didn't give them what they want. Other times, people say that you haven't given the benefit of the doubt because you've complained about treatment by a member of a higher-privilege group and as such, the speaker gives the person you complain about the benefit of the doubt instead of giving it to you.