The bug

Commands which requires only a single entity print the error message

Entity '%s' cannot be found

when a selector matches multiple entities.
The expected error message would be something like

Selector '%s' matches more than one entity

How to reproduce

Use the following command when multiple entities are in the world

/tp @p @e

Code analysis

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.