It takes effort to evaluate examples. Revealing that one example is bad raises the possibility that others are bad as well, because the methods for choosing examples are correlated with the examples chosen. The two obvious reasons for a bad example are:

1. You missed that this was a bad example, so why should I trust your interpretation or understanding of your other examples?
2. You know this is a bad example, and included it anyway, so why should I trust any of your other examples?