A few years ago I had sold equities for the first time ever in my life. I received them from my employer as RSUs. And I had paid taxes on the shares when they vested.

When I reported the capital gains (as a good citizen does) I didn't realize the cost basis was set to $0. So I greatly overpaid on the taxes. I had trusted HR Block to take care of that for me. But the fault was mine, not HR Block. The cost basis was reported on a different supplemental form that I wasn't aware of.

But then the broker gave a webinar on the whole process, so I was prepared for subsequent years.