It is damned near impossible to counterfeit Canadian bills effectively.......except that nobody really looks.
take out a bill.
Down the right side is a reflective strip that shows maple leafs wheb turned in the light.
On the left side, beside the large denomination numbers, there is a small set of numbers (the denomination) you can only completely see when the bill is held up against a light.
Also, just to the left of centre of the bill, the face of the person appearing on the bill also appears in miniature when the bill is held up against a light.
The stores would not have a problem with counterfeits if they gave their clerks a 10 minute instruction on detecting them......and hired enough so they have time to actually LOOK at the money.