While it's not quite pop culture, and we're not sure if it's a joke or not, there was a common Medieval motif of painting snails in battle in manuscript marginalia. /u/Sunagainstgold has a thread about it (read her links, but also down her comments). There was a similar thread asking about murderous rabbits that went unanswered.

Often times, though, is there a clear difference to us as observers between an in-joke we don't understand or, as /u/whoosier says here, "just the artist having fun"? It's a question that we can't really answer. The anthropologist Clifford Geertz talks about how the anthropologist must be able to distinguish between a "blink" and a "wink", something that we'd know only from cultural context. That is, the anthropologist must be able to understand the symbolic systems that make up culture he's studying (to Geertz, culture is always a "system" made up of different symbols; "Believing, with Max Weber, that man is an animal suspended in webs of significance he himself has spun, I take culture to be those webs, and the analysis of it to be therefore not an experimental science in search of law but an interpretive one in search of meaning" is his famous quote). A wink and a blink could look the same to an observer who doesn't know to distinguish the two gestures, but mean very different things to the person making the gesture.

Here, it seems, most medievalists think the people who paint these illustrations were "blinking" by making just random funny pictures (maybe in imitation of other funny pictures they saw one time, or two people thinking of the same ridiculous image), but we can't really say for certain there's not some "wink" here where there's a deeper meaning to jousting snails or tumbling men or murderous rabbits. But in general with "in-jokes", if we don't understand it from the start--if we never learned that there are both winks and blinks--there's a good chance we might not even realize an in-joke was there at all if we can't already understand it.