I start this with a learning experience. Always have a few business cards on hand, even if you think you won't need them. As someone who normally didn't carry cards around and as someone who is working to get their own business started, I missed an opportunity to hand out my cards to a few people last night at a local art show. I'm a polyglot software engineer that is working on a 3D printing business plan and didn't really expect to meet some folks at the event who would be interested in what I'm working on. So, even as an old boyscout, I wasn't fully prepared for the interest and the opportunity to exchange business cards with possible clients.
I will have a few cards with me no matter where I go now because a missed opportunity could mean a loss of business and that's a loss I do not want to miss out on again.