If you have not met your reporting obligations, whether you were aware of them or not, they can impose huge fines. Many american citizens here, or those with american parents, fear crossing the border or drawing attention to themselves.
Lottery winnings are not taxed by Canada, but if an american wins the lottery anywhere in the world, the USA will take its chunk.
And to add insult to injury, if you want to get out and give up your US citizenship for good, it will cost you. Before 2010 renunciation was free. From then to 2014, it was $400. Now they charge $2,350 plus a big exit tax if your net worth is above $2m.
The US is the only country that taxes its overseas citizens this way, beside Eritrea.