Medic view
I nearly got caught in this but was of an age that I could retire and return. The calculation is so complex that you almost need someone with a degree in maths to understand. One year, the calculation showed that I had contributed more than I had earned. I had applied and obtain a discretionary point for providing a service above my basic work. I also moved one step up the salary scale. The contribution into the pension involves something like the difference between last years salary and this year multiplied by 16. This is then added to your own and employers pension contributions. You then pay tax on the pension contribution above £40k. The things that have also caught people especially medics is taper rules. Your salary before tax including your pension contributions plus your employers pension contributions is your total rewards package. Once this goes above £110K (I think) then the amount you can contribute into your pension tapers from £40K. If you are lucky enough to have a total rewards package above £210K then the max you can put into a pension is £10K and you get taxed on contributions above these figures (55%) The problem is that you do not find out the contribution until October of the following tax year! A good IFA is vital.
This is why the NHS is in a mess and needs expensive locums (as reported in the papers today). The vast majority of my Consultant colleagues do not do extra paid work and are asking to reduce the hours they work