Based on your financial expertise, provide your response or viewpoint on the given financial question or topic. The response format is open.

Withholding for unexpected Short-Term Capital Gains and Penalties
Assuming U.S. law, there are "safe harbor" provisions for exactly this kind of situation. There are several possibilities, but the most likely one is that if your withholding and estimated tax payments for 2016 totaled at least as much as your tax bill for 2015 there's no penalty. For the full rules, see IRS Publication 17.