I understand it is illegal in France, heard some French doctors setup shop in Northern Italy on the weekends and work for cash to keep food on their tables.
Back in the early 80's. I worked in Vancouver Canada. At that time, docs incomes were capped. If they had a good practice, by November they earned as much as they were allowed for a calendar year.
So they either practiced for free until January, or closed the doors for a month or two. Those who closed often would drive down to Bellingham WA and work the ER's. That income was not taxed in Canada (their tax rules) and often covered 50% of the gross they made in Canada in the 10 or 11 months they worked there.