It's not about the bad news itself. It's about whether your boss heard it from you first, or found out from someone else and had to wonder why you didn't say anything.
Of all the unwritten rules of office life, this might be the one that costs people the most credibility, the fastest, without them ever realizing why. A boss who's blindsided in a meeting by a problem you already knew about doesn't just react to the problem, they start silently reassessing whether they can trust your updates going forward. That reassessment is quiet, and it's expensive, and it's almost entirely avoidable with a two-minute heads-up sent before the meeting instead of during it.
The rule isn't "never have bad news." Bad news is normal and unavoidable. The rule is: your boss should never learn something important about your work from anyone other than you, and never in a room where they have to react to it live, in front of other people, with no preparation.
You know a client is going to raise a complaint in tomorrow's leadership sync. Instead of letting your boss hear it live: "Heads up before tomorrow's sync, the client is going to bring up a delay on their side of the project. Here's what happened and what I'm doing about it, so it doesn't catch you off guard in the room."
That message takes ninety seconds to send and changes the entire dynamic of tomorrow's meeting. Your boss walks in already knowing, already has a response ready, and, critically, already trusts that you're the kind of person who gives them that warning. Without it, they're improvising in real time while everyone watches, and some of that discomfort attaches itself to you, fairly or not.
This applies to more than client complaints. Missed deadlines, a team member's resignation, a budget overrun, a mistake you made yourself, any of it. The instinct to delay telling your boss, hoping the problem resolves itself before it needs to be mentioned, almost never pays off. It just shrinks the window your boss has to actually help, and it guarantees that if it does surface elsewhere first, the story becomes "why didn't they tell me" instead of "what happened."
The version of this that costs the most: your boss's boss finding out something from someone other than your boss, because your boss didn't know either. That's the surprise that travels furthest and does the most damage, and it's entirely preventable by making "tell your boss first, even when it's uncomfortable" a habit rather than a judgment call you make fresh every time.