Have you ever accidentally necroposted?

I've literally just had to google necroposting!! I had no idea this was a thing, and I still don't really understand why it's bad? Doesn't it just mean that someone's found an interesting post that they feel is relevant enough to want to comment on, which saves starting a whole new thread on the same thing?
I think I must have necroposted, and not even realised it had a name... or that it was bad... :/ Every now and again I get hit with the thought that I'm getting old! :# :D