Missing commas in tuples results in implicit string concatenation. Probably not what you intended to do.

Implicit string concatenation that resulted from a typo can change the behaviour of the application. Take for example:

def is_positive(word):
    words = (
        'yes',
        'correct',
        'affirmative'
        'agreed',
    )
    return word in words

is_positive('agreed') is True  # evaluates to False

is_positive('agreed') evaluates to False because a typo resulted in the comma being missed from the end of 'affirmative', resulting in 'affirmative' and 'agreed' being implicitly concatenated to 'affirmativeagreed'.