What are the tax consequences of not having any Federal Income Tax withheld from my pension?

[VolvoGirl]

You might have a tax due on your tax return in April.  And if you end up owing too much there is an underpayment penalty for not paying in during the year.  

[tagteam]

You may owe federal income tax when you file your income tax return unless you make estimated tax payments. You may also owe a penalty for underpayment of estimated tax (see below for information about avoiding the penalty).

You can avoid a penalty for underpayment of estimated tax if you owe less than $1,000 in tax for 2019, or if you pay at least 90% of the tax owed for 2019 or 100% of the tax shown on your return for 2018, whichever is smaller. 

See https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes