The most common reason for retired persons to see an amount added as Other Employment Income on Line 10400 is: 

• You received a T4A - Statement of Pension, Retirement, Annuity and Other Income with an amount in Box 118 - Medical Premium Benefits,
• or Box 119 - Group Term Life Insurance Premiums, they will show up on Line 104 of your tax return as "Other Employment Income". 

For a complete list of amounts that may be included on this line, please see this CRA information: Line 10400 - Other employment income

My wife is not retired, does not have a T4A or any other income but still a ridiculous amount of $99.90 appears on line 104. Have spoken to Turbotax but they have no idea how it got there. CRA also have no record of it. This error cost me $23.59 in additional tax. Turbotax should reimburse me.