Basically, if you don't have sufficient taxes withheld from your W-2 pay, then not only will you owe taxes at tax filing time, but you'll also be assessed an underpayment/under withholding penalty on top of that. It would appear that's precisely what happened to you, based on your post at https://ttlc.intuit.com/community/investments-and-rental-properties/discussion/only-509-taken-out-of...
