I learnt this the hard way lol
I'm usually a vey polite, smiley person by nature, so if someone smiles at me or says 'good day' I always reciprocate.
I used to frequent a local sandwich shop and one of the workers also attended my uni. He was a pakistani guy on a student visa, and we always had a bit of a chat about work/uni etc. Anyway I had never told him I was married (didnt really see the point it wasnt like he was flirting or giving any signals, so I assumed he realised it was platonic) and whenever me and my husband were in the shop together he was never there.
One day my husband and I were walking through the local park and I saw him playing cricket, I gave him a wave but he kinda gave me a dirty look but i didnt think much of it.
Next time I went into the shop and ordered a baguette, this guy started some all out attack on me infront of all the customers, claiming I was a slapper and a whore for chatting to him when I was married. He even said he had been speaking to his mom about me and that he had considered asking me to marry him (WTF). I was really shocked and embarressed, I promptly cancelled my order and have never been back to the shop.
I told my husband and he thought it was soo funny, I was quite shook up and supprised that general conversation could be so misinterpreted. Now I never speak to FOBs and if I have to I give them lots of dirty looks.