The bug

The message which appears when you specify an non-existent team as value for the Team tag of an entity is logged as "Info" instead of warning or error.

[Server thread/INFO]: Unable to add mob to team "NonExistentTeam" (that team probably doesn't exist)

How to reproduce

Summon an entity as member of a non-existent team

/summon armor_stand ~ ~ ~ {Team:"NonExistentTeam"}