Summarize:

You may be tempted to hold off on delivering the bad news because you don't want to be blamed for the bad news, even when it's not your fault. However, delaying will often make the problem worse. For one, if your boss finds out that you didn't tell them, they're going to be even more unhappy. Also, if your boss has the bad news, they'll be able to prepare for the consequences instead of being blindsided by them, which means the company's reaction as a whole can be more positive.  Any problems with significant consequences should be reported as quickly as possible. Assess the potential impact of a problem to determine when to report it. Any issue that may result in danger or law suits, such as major violations of company policy, should be reported immediately. The sooner you get the information to your boss, the sooner they can take it to their boss, if they're not the head of the company. If the bad news is not urgent, keep your eyes open to see when your boss seems less busy. When you notice their workflow slowing down a bit, go in and ask if you can talk to them. That way, you give them a chance to push it off to another time if they don't have the time right now. However, an important issue, such as sexual harassment, should be reported immediately, regardless of how busy your boss is.  For less pressing news, you could pick a time when you know the person will be in a good mood, as then the bad news won't come as such a blow.  For instance, you could say, "I have something I'd like to discuss with you. Is now a good time?" Also mention about how long you think it will take. You definitely don't want to catch your boss as they're trying to get out the door or running downstairs for lunch. You could also send an email asking about a good time if that's more your boss's style. Trying to soften the blow with rhetoric is not going to help. Just get to the point. Your boss is busy, and most busy people prefer directness, as it takes less time and they don't have to guess at what you mean. Therefore, have a concise statement to open the conversation with.  For example, you could use a statement like, "I have some problems with the new line." Phrasing the statement that way reflects ownership and the acceptance of responsibility. If you blame others, you may be viewed as a whistle-blower or company rat. On the other hand, if you know your boss likes small talk, it can help soften the blow to chat a bit first. Your tone sets the mood. If you treat the problem as something that can be fixed and you remain positive, your boss will likely see it that way, too. However, if you're all doom and gloom and come in with a depressed posture, your boss is going to pick up on that, too. Make sure you stand up straight, look your boss in the eye, and speak in a clear tone of voice. That shows you have the confidence to solve the problem.
Don't wait. Find a good time to talk. Get to the point. Pay attention to your tone.