I fully agree and see it constantly in my business (accountancy). I have self employed and small business clients at lots of different tax thresholds who are deliberately holding back their potential and deliberately working less than they could, to avoid breaching tax thresholds.
Right from the lowest level, i.e. part time ebayers, mobile hairdressers, office cleaners, market traders who are just working "enough" hours to qualify for UC or tax credits, free prescriptions, rent allowances, council tax discounts, etc - they won't work more because the benefit of a few extra working hours doesn't make up for the loss of benefits.
At the other end of the scale, I have a dentist who has a hard time keeping his income under £100k as he doesn't want to lose free childcare and doesn't want to pay a marginal tax rate of 63.25% on his extra earnings over £100k, so he's actually cutting down his working hours and doing less work to keep his income under the £100k.
Numerous different types of small businesses who keep their profits under £50k so they don't lose their child benefits at the same time as going over the higher rate threshold - they could work harder and grow their business more, but make a conscious effort not to.
But the worse tax cliff edge is the £85k VAT threshold where you can often be worse off by thousands of pounds by letting your sales/turnover (not profit) go over the £85k by a small amount, i.e. they could be £10k worse off if their sales grow from £85k to £86k if they are in retail, hospitality, a small garage or in fact any small business dealing directly with the general public. It's completely immoral and stupid to have that kind of "brake" on a small business that could otherwise grow! I've just had such a case, a small guest house who have had to reduce the number of letting bedrooms as they were too close for comfort to the £85k, so couldn't take the risk and the only safe way is to reduce their capacity, i.e. hold back a business that otherwise could have grown! It's madness.
The marginal tax rate graph (i.e. how much extra tax you pay on an extra pound of income) is like a mountain range with peaks and troughs, whereas common sense says it should either be a straight line or a gentle upward sloping curve. Our current crazy system has marginal tax rates at certain income levels of 60-70-80% which is a massive disincentive to push yourself and earn a bit more. Worse still, in several circumstances, it can be over 100%, in fact in some cases, can be hundreds or thousands of percent (such as the VAT threshold breach).
We desperately need a radical reform of the tax/benefits system to end this stupidly damaging system which has been created by a couple of decades of politicians tinkering with the tax system and being incapable of understanding the effects of their constant meddling.
Working harder/working more should always be worthwhile, wherever you are on the income scale. Quite simply any point on that scale where you end up "taking home" less than half of the extra you earn (i.e. marginal rate of over 50%) needs to be corrected and it's a massive disincentive to work harder/more if you know you're going to end up with less than half of your extra earnings in your pocket.