Once and I was slender at the time so I wasn't insulted. It was the dress. There was a style at the time that looked a lot like a maternity dress. I was working at a bank and this customer asked me what I had...and I told her I had not been pregnant and the funny thing was that it was like she didn't believe me. She looked really doubtful.
Another time, I was a little heavier but not really fat, and I was slouching in my chair at the office, and a customer (again in a bank) said to my boss, "I didn't know Pat was pregnant." My boss just looked at him like he was nuts, and said, she's not.
I looked down at how I was sitting and it really stuck things out in a very unflattering manner and I have to say it was a good lesson to me in posture and remembering where you are at all times. I ignored the comment but sure cleaned up my act.