I have noticed that the program is double-counting the Guaranteed Income Supplement amount, so this could be the reason why the original poster's @Beedas OAS is triggering an error. When I report the "taxable portion" from box 18, the program forgets to reduce line 113 by the supplement amount (as it has done in the past). The result is that total income on line 150 shows the supplement amount twice. This needs to be fixed.
