For decades this was the harshest penalty in the retirement code. Fifty percent of whatever you failed to withdraw.
Not fifty percent of the tax. Fifty percent of the money.
Miss a $20,000 required distribution and the penalty alone was $10,000, before you paid any income tax on the withdrawal itself.
That has changed, and the current rules are dramatically more forgiving. But the fix requires you to do something specific, and doing nothing is still expensive.
Here is the current penalty, the discount for acting fast, and the waiver process that the IRS grants far more often than people expect.