There are a number of ways denial of counsel of choice can come up, most frequently related to the availability of civilian counsel. Here’s an interesting one.
United States v. Gonzalez-Lopez, 548 U.S. 140 (2006). The opinion is written by Justice Scalia. Here are the important part of the opinion:
In sum, the right at stake here is the right to counsel of choice, not the right to a fair trial; and that right was violated because the deprivation of counsel was erroneous. No additional showing of prejudice is required to make the violation "complete."