This is one of the reasons why I'm always interested in how people define what "single" is.
Some people define single as not being married.
So if their date at the party happens to walk into the next room and isn't right there at the time... They'll tell the person they're talking to that they're single--and give off the impression that they're also available.
Or maybe a person has met someone online and spend vast amounts of time on the phone or online with someone for the past 2 months... And one person might think they're in a relationship--but the other person, upon meeting someone else, insists, "I'm single."
One of the toughest things I've encountered in the modern dating world is trying to find out if the person you're interested in talking to is actually single and available to date in the first place.
I'm sure this has always been part of the challenge, but for all the information that's out there, it seems easier for people to fudge their relationship status/identity now more than ever.