When you omit break, the program continues to the next case's code. I'll show you the rare situations where this is intentional and how to make it clear. You use fall-through to group multiple values with the same behavior: case 'a': case 'A': cout << "Letter A";.
Both cases run the same code. If fall-through is intentional, add a comment like // fall through so readers know it's not a bug. Most of the time, missing break is an accident that causes wrong behavior.