When I was working, any bonus I got had some taxes withheld from it, so it didn't affect my April tax bill because the taxes paid side kept up with the income side.
But when I received an investment income bonus (i.e. cap gains distribution), I usually had it automatically reinvested. So, not only did I have no taxes withheld from it, I also didn't have the equivalent cash from it sitting around in my bank account to pay estimated taxes. This created an imbalance when April rolled around.
This time, however, unlike last year, I will be taking a large, year-end cap gain distribution in cash. While I have other uses already set up for some of the money, I will allocate some of it toward estimated taxes in early January to lessen the blow in April, probably paying about 2/3 of the taxes (state and federal) in January, allowing me to easily be in a "safe harbor" for each and avoid any penalties.