Had it happen one night in a nightclub. I was just standing, idly watching the dancefloor when I noticed this woman in her mid 20's looking in my direction. Didn't think any more about it until a couple of minutes later I noticed that she still seemed to be looking fairly intently in my direction (and mostly well below eye level!). Now I'm a bit short-sighted (& too vain to wear glasses for going out!) and wasn't sure if I recognized her from somewhere, but didn't want to stare. Next thing I know, she's walked over and is standing in front of me, slightly to the side and is backing her backside into my crotch - I jumped back slightly (a bit shocked) and when she turned around I realized that I'd had a one night stand with her friend about a year previously and remembered that the one I'd had the sweaty night with had commented that her friend would be jealous as she had a real thing for 'big' men.
At that point my wife (who I'd only been seeing for about 6 months at that point) came back from dancing and the woman looked a bit embarrased, then wandered off. Of course my other half found my unease at what happened highly funny!
After having things like that happen I can now understand why my wife sometimes gets quite pissed off by blokes staring (like unsubtle gawping, not the complimentary extended glance) at her when we've been out - she's got an impressive chest and what I'd describe as an 'amazon-like' figure - rather than being the compliment some people seem to think. It's pretty annoying to be just though of as a walking body part that happens to have a person attached (although no-one's ever talked to my dick the way some blokes seem to just talk to her chest)