Our own Amy Ratcliffe noticed something about Firefly that you probably weren’t aware of (It wasn’t even revealed in the recent book Firefly: Still Flying—as far as I recall). Han Solo in carbonite pops up everywhere.
Apparently, the props team tried to inject the figure wherever they could as a nod to Nathan Fillion’s Star Wars fandom. Over time, it became something of a running joke—so much so that there is a rumor that one is hidden in each episode.
Amy didn’t catch them all, but you can see a few more examples on Geek With Curves.