def complement_strand(sequence):
    """
    Returns the string which will be the second strand of the DNA sequence
    given that Ts complement As, and Cs complement Gs. If given
    a bad input, the function returns "Sequencing Error"

    :param sequence: A DNA sequence
    :return: the complement string for the DNA sequence
    """

    complement = ""         # This can be used to "build" the complement

    letter_dictionary = {"A": "T", "C": "G", "T": "A", "G": "C"}
    for letter in sequence:
        if letter in letter_dictionary:
            complement += letter_dictionary[letter]
        else:
            return "Sequencing Error"

    return complement