Di Matteo tried to tack an extra euro onto the bill, an annoying though possibly honest error. 这个句子是由下面句子缩减而来：

= Di Matteo tried to tack an extra euro onto the bill, which is an annoying error though  it is possibly an honest error.

由此可知，though并不是并列连词，而是引导让步从句的从属连词。