The bug

A mob spawner with invalid entity data spams the log:

Failed to parse entity custom name [{malformed}]
...

Expected would be that the spawner refuses invalid spawn data on creation / update or that the warning is only logged once.

How to reproduce

/setblock ~ ~ ~ spawner{SpawnData:{entity:{id:"minecraft:armor_stand",CustomName:"[{malformed}]"}},MinSpawnDelay:5s,MaxSpawnDelay:5s,SpawnCount:1s}

The log is spammed with the warning.

Do not forget to break the spawner afterwards again.