People who are vulnerable to criticism should not criticize others, especially not for the faults that they themselves have (since such criticism will likely be returned). He's been hounding his opponent for dodging taxes, when it's public knowledge that he still owes the IRS for years of tax delinquency. People in glass houses shouldn't throw stones! A: "She just spends her time complaining about other people. It's so shallow and annoying!" B: "Now, now—people who live in glass houses shouldn't throw stones."