The bug

Using selectors which can include multiple entities as value for a command argument which only allows a single entity shows the following error message, despite all entity types being allowed.

Only one player is allowed, but the provided selector allows more than one.

This appears to affect only some commands, for example /execute if score @e correctly says "entity".
Likely the wrong exception is used, argument.player.toomany instead of argument.entity.toomany, see bp.class (17w45b).

Affected commands