In my Runequest campaign, the players were in a weird volcano dungeon and encountered a Gelatinous Cube to their surprise; they weren’t used to seeing D&D monsters in Runequest, to say the least. They killed it, and then entered the room at the end of the tunnel — there they found a huge pile of filth which proved to be occupied by an Otyugh, which they also managed to kill. During the fight I rolled for “wandering monsters” (not a Runequest concept) and an owlbear-griffin hybrid stormed in, which the players immediately dubbed a “growlbear”. Each round of combat, the growlbear attacked with a different appendage – stinging tail, beak peck, claw, tentacle, wing buffet, etc.

I figured such a hybrid would be a mess. Well the players were tough and they weathered that one too. When they returned to the all, there was a Roper standing there. 

This is when they started to Think. “Why is this place infested with all the most retarded D&D monsters?” “Wait, how fast is a Roper? How did it get there?” Alas the penny dropped, and they realized that all these entities must be illusions, created by some powerful force (an undead wizard, as it turned out). And of course they were right — but at least I had a lot of fun using the stupidest possible critters vs. players who thought they’d escaped that kind of thing by converting to Runequest