Some things are better left unsaid, a wise girl once told me. Actually, five regretful girls told me—after saying things to their bosses they wished they could have sucked right back in their mouths (if only their feet weren’t blocking the way).Learn from their mistakes, and avoid these topics with your manager at all costs.1. Your side businessIt’s fine to have a hobby, and it’s fine and dandy if that hobby makes you a profit. What’s not fine is selling your services to your manager and her peers.Case in point: A girl I used to work with was getting some negative feedback on her recent performance. Little things were building up to be a big thing, and rumors were swirling that management had had enough of her lack of attentiveness in meetings and constant mistakes.They finally found out why this initial rock star was dwindling into a clock-watcher: At an important client dinner, she announced to a manager (who just told the table that she’s getting married) that she’d love to plan her wedding because she’s been trying to build her clientele. Turns out, she was an on-the-side event planner.When she asked me later if I noticed how weird managers were being around her, all I could think of was “Yes, and it’s probably because they’re paying you $50K a year and their work is being neglected by the start-up you’re promoting on their time.” Yeah, that might be it.2. Exhaustion due to late night studyingIt’s totally cool to walk into the office tired from a late night of working overtime—in fact, it will probably earn you some big-girl Brownie badges that’ll help for your next promotion. But it’s not OK to complain because you had to stay up and study for an exam you have to take in order to make a career switch.True story: a girl I worked with in a marketing department walked into a morning meeting yawning. When our boss playfully asked if she had a late night out, the girl replied, “Ugh, I wish. I was studying forever last night for my teaching certification.”If you’re wondering if she pretty much told our bosses that she’s actively trying to get another job outside of the company—well, yes she did. And, she felt about 10 times more awkward than you do reading this after she broadcasted her calculated plot to quit ASAP as soon as she could find a good teaching job in a nice school district. She ended it with, “It’s just so hard finding a job in this economy, you know?”3. TMI Doctor visit detailsIf you need a sick day, fine. Even feel free to share whether it’s something minor or serious, if you really want. But, whatever you do, leave out anything that will conjure up unpleasant images in your boss’ mind.A few years ago, a girl I interned with sent a note to our male boss about an infection she had, ahem, down there. I’ll never forget the look on his face when he opened up the email. It’s the same face you make when you see intoxicated couples making out on the subway. You think, they’re clearly not in their right minds, and they will regret this and feel as appalled as I do tomorrow when they come to. And so will you if you divulge things to your boss that should never be discussed outside of an exam room.4. How bad you screwed upOnce at a work happy hour, a group of us were sharing stories—you know, funny happenings around the office, G-rated embarrassing moments—until one admission turned the room silent. Turns out, it’s one thing to accidentally break the copier, but quite another to damage client relationships.One of my peers had a cocktail in hand with pink liquid swishing from side to side as she laughed so hard she cried about the time she accidentally tweeted about a Bravo TV show on behalf of the client on accident. As soon as she saw our manager’s agitated fluttering of the eyes, she panicked—and ordered a bigger drink to wallow in her sorrows. She was a social media manager, and I don’t blame her for wanting to swim away in that fruity drink of hers. If I had offered up a reason for my bosses to no longer trust my judgment, I would have ordered up another round for myself, too.5. The real reason you left your last jobAcceptable reasons for leaving your previous job for your current one can be any number of things: relocation, a career switch, an urge to feel challenged or an interest in a different company culture. One thing that is not a sufficient reason, and if spoken could cause some serious damage to your boss-employee relationship? That you were looking for something “slower-paced, less challenging, easier.”That is a direct quote from a gal I worked with who was apparently surprised that our manager didn’t clutch her pearls in flattery and thank her for the compliment. I mean, this woman helped to build the business from the ground up—and a new hire is telling her that she essentially downgraded by working here because she needed a break from the hustle and bustle of a tougher company? Even if it’s the truth, this is essentially the office equivalent of one coattail-holder slapping the coat owner in the face.While outright lying can harm your relationship with your boss, the same goes for being way too honest and open with things you should share only with your friends (specifically, your good friends—the ones who won’t judge you). Even if you’re close with your manager, it’s a good rule of thumb to run from the above situations just in case. Now you know what happens after a bad case of word vomit: Even a seasoned custodian can’t clean that career mess up.