As indicated by the below video from the Jimmy Kimmel show, this failure to be honest can end up making people look like idiots. It seems that being in the know about music is much more important than just knowing what you know and enjoying it.
The best approach when presented with a band you've never heard of before is to be humble... admit you don't know them... and then if they are good, be honest when you share their music that you just heard of them, even if their album came out years ago. If you don't, you might end up looking like these people:
In order to practice what I preach, here is a music video from two years ago that I just found out about yesterday.
The band is Danger Beach out of Australia and the video is for their 2011 song "Apache." It's a whimsical animated video lasting only two and a half minutes and perfect for a Sunday afternoon viewing.