I bet moms with small kids are one of the top groups to unintentionally show their goods. I have some stories to prove that... But it can happen a bit differently too.
Last summer I was picking up my child from daycare and I arrived quite early so all the kids were still inside playing. Boys were playing with cars and their nanny, brunette in her late thirties, was also crawling on all fours on the floor with them. She's pretty good looking, definitely not more the B-cups on her. I sat down because I wasn't in a hurry. When he turned to my direction I lifted my eyes and was greeted with superb downblouse view - both her breasts were on display, all the way to her brown pointy nipples! Obviously she wasn't wearing any bra. She just chirped "hi!" and continued the play, and I still got a couple of good looks of her titties from different angles - not to mention pretty enjoyable view of her ass in her pretty tight jeans...
She never showed any sign of realising what she had just shown me.
No question what my fantasies were about for a good while after that!