When lowering the players walk speed the player's screen seems to have a chance of failing to render. In which case you just get a blank backdrop of the void/sky, and your gui.

This is used with a bukkit server which modifies the players walk speed.

I lower their walk speed gradually until they can no longer move.
They don't crash and there is no error log. But they can no longer see any blocks, entitys, their hand etc.

This persists until you restart.