"That's what she said!" is, somewhat unfortunately, one of my favorite jokes.
Although my mom is very smart and I got a good part of my sense of humor from her, she found my seemingly random outbursts of "That's what she said!" to be very confusing.
And then, I explained it to her... and then she said it in a doctor's office, and in front of my nephew.
And the lesson is: There's a time and a place for "That's what she said!" and some people just can't be trusted with it.