Commands which requires only a single entity print the error message
when a selector matches multiple entities.
The expected error message would be something like
Use the following command when multiple entities are in the world
Based on 1.11.2 decompiled using MCP 9.35 rc1
The method net.minecraft.command.EntitySelector.matchOneEntity(ICommandSender, String, Class<? extends T>) returns null if multiple matching entities were found. It might be better if it threw a CommandException instead.
Note: The method matchOnePlayer might want to rethrow a more specific exception.