The KafkaProducer.send function has a try/except clause that hides the exception in a custom serializer. This makes finding bugs inside custom serializer difficult.
Example code:

from kafka import KafkaProducer

def raiseError():
    raise NameError

producer = KafkaProducer(bootstrap_servers="localhost:9092",
                 value_serializer=lambda m: raiseError(m))
producer.send("topic", "value", "key")

This program will not raise error even though it should. Adding except NameError: in KafkaProducer.send can solve this no raise issue, but I am not sure if this is a good idea.