The first map for Medusa's Island [places a jellyfish on land with line 107](https://github.com/NetHack/NetHack/blob/NetHack-5.0.0_Released/dat/medusa-1.lua#L107) - it's pretty minor, but moving it one square up or to the side solves this.
The first map for Medusa's Island places a jellyfish on land with line 107 - it's pretty minor, but moving it one square up or to the side solves this.