In all of my Minecraft experience to date, if a mushroom island is found out in the open ocean with no bordering biomes, it will be a one-biome environment of only mycelium. However, today I encountered a mushroom biome with ocean biome encroaching onto the actual land sections, resulting in grass generation alongside the proper mycelium. Not only is this unsightly, it results in the normally spawn-free mushroom islands having squares where hostile monsters can spawn at night/in darkness.
For an example of this, create a new Default world with the seed -2131116689 and teleport to -8203 63 608. The faulty generation will be visible directly under your feet.