As a tax preparer, I've seen yet another often overlooked error.  An employee starts to receive CPP and elects to stop making CPP contributions (age appropriate), but continues to work with the same employer.  The employer will often continue to make CPP deductions....   when they finally (in this case after three years) stop deducting, they will continue to put the CPP pensionable earnings in box 26 on the T4 slip at the full amount, instead of zero.  (Makes it also tough to complete Schedule 8, and the employer must then cancel the original T4 slip and submit a new one with zero in box 26.)