Well, you can be surprised. It's surprisingly common, in my experience, to believe people who pretend they are on your side. One interesting and typical case that is documented through countless online videos is police interrogations, where the interrogator is usually an expert in making it seem he (or she) is on your side, despite how obvious it should be that they're not. "Can I get you a meal?", friendly tone, various manipulations and before you know it you've said things that can and will be used against you whether you are guilty or not.
And you don't get the meal, either.