Are you playing the audit lottery? Have you told people that if they pay you with currency you will give them a deal and not charge HST? Have you told anyone that paying taxes is for suckers and you are not a sucker? CRA receives plenty of calls from people who have heard these stories and want to let CRA know about you. CRA will eventually come check on you if they get tips.
The other way people play the audit lottery is by claiming expenses without having the receipts. If CRA asks for the receipts, and you cannot provide them, the expense will be disallowed and interest and penalties will be charged. Another popular gamble is to claim vehicle expenses without keeping a mileage log.
I recommend that people have receipts for everything they claim on their tax returns and of course, that they declare all their income. If you are committing tax fraud, don’t tell anyone.