I will never forget the time an acquaintance showed me her prior year's return, which had been prepared by a well-known nationwide tax preparation company who shall remain nameless.
She had worked part of the year in an office where they paid her as an independant contractor (even though she should have been treated as an employee, but that's a whole 'nother story). They withheld nothing and gave her a 1099 for the earnings.
So her return shows income from this job of about $9,000, mileage of about $12, supplies of about $37, and postage expense of exactly $2,000. When I asked her about the postage being so large in relation to the income and other expenses, she said "they just made that up so I wouldn't have to pay as much."
Sheesh! If you're going to cheat, at least try not to be so obvious!
As for me, back in the far recesses of my mind, I think I remember earning some cash that I didn't claim on my tax return back when I was young and rebellious, though I can't even remember exactly what it was for and I don't think it was more than $100 total.
I'd never cheat now, and not when I was working. So not worth it when you could lose your CPA license for it.
I've also refused to prepare returns for clients who admitted that they had income they weren't planning to report.