As is often the case, sometime last

As is often the case, sometime last month I walked on the street and found someone smiling or even trying to talk to me, but I couldn't recall who he or she was at all. I felt so ill at ease that all I did was only to gently stand there and try my best to smile as brightly as possible.
One day such a thing happened again. While I was greeting that beautiful girl, a crazy idea came into my mind that I decided to keep on talking with her though I didn't know her. Luckily, we had a pleasant conversation for ten minutes. But when saying good-bye to each other, we both suddenly found she had mistaken me for my elder sister. What an embarrassing experience it was!