Lack of planning! In most cases, people tend to first immigrate to the United States, obtain visas, become tax residents and only then think about the taxes. The correct, the indispensable way is to analyze and do fiscal planning before immigrating, in order to organize a consistent set of actions, strictly within the law and exploring possibilities allowed by the authorities. Real estate, patrimonhy, family situation, succession, investments… All of this must be assessed and planned for.
Another common mistake is for the tax resident in the United States to think they must only declare to the IRS what they own in the United States, thus not reporting what they own in Brazil, as well as the respective revenues from Brazilian sources.
Some Brazilians, when arriving in the United States, also tend to make mistakes in regard to the definition of the type of business they are starting, which in some cases ends up generating a higher tax burden.