No one's job is easy, even when things are smooth sailing.

But when your job starts to feel more challenging out of the blue, it may be time to sit up and pay close attention. Lynn Taylor, the author of Tame Your Terrible Office Tyrant: How to Manage Childish Boss Behavior and Thrive in Your Job, told Business Insider that the savviest professionals always keep an eye out for the classic signs that their job is in danger.

If and when you notice red flags popping up, you can attempt to turn the tides before it's too late. That said, it's not always easy for employees to pick up these signs. "What's important is to be alert of situations around the office to ensure the security of your position in the company," Ryan Kahn, a career coach and author of Hired! The Guide for the Recent Grad, told Forbes. Here are the most common signs that you could be walking the line between hired and fired.

You've received a poor performance review:

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Most people on the job are subject to an annual review, that awkward one-on-one meeting with your boss where they're obligated to offer you constructive criticism. Most bosses won't turn down the opportunity! But if you find yourself leaving the meeting feeling completely slammed with a whole wheelbarrow full of unexpected complaints, it might be time for a reality check. "Critical performance reviews could be a major sign that your job is in jeopardy," Kerr told Business Insider. If this happens to you, it might be a great idea to start looking around for other opportunities before you need to. Meanwhile, try to stay calm and work towards making improvements in the areas that were brought to your attention.

You've been left out of the loop or removed from projects:

When your colleagues are working on new projects, or attending meetings that somehow never made it onto your calendar, you'd better pay more attention to the dynamics. Keeping you out of the loop is often one of the biggest signs that trouble's around the corner. "The more distance, the better--and since they [colleagues] know you'll soon be gone, they want information to stay proprietary," says Taylor.

You sense tension with the boss, or with other employees:

Work environments can feel like families—there's no shortage of competition, camaraderie, and casual banter. Think of what your "baseline" for good relationships with your boss and your colleagues would be. Has that deteriorated into petty digs and heavy sighs? If you feel like your colleagues are doing anything and everything to limit communications around the office—whether personal, or work-related—it might be time to polish up your resume.

Written complaints about your work are piling up:

Too many emails detailing small work issues can add up to a big problem! pngimg.com

Your email has blown up with the strangest things lately: "Hi X, I've noticed that the latest memo you sent out had a couple of mistakes that I thought we had cleared up. Just FYI!" If the FYI's are mounting up in your inbox, your boss and colleagues could be working to build a case against you. Or, not. This isn't a marching order to feel paranoid, but rather to take a serious look at written communications between you and your colleagues that are changing in tenor from easy to terse, with a whole new level of specificity. Also, be mindful of who's being cc:d on these communications. If you notice that supervisors are being brought into the complaint loop, it's important to pay attention.

Request for details about expenses and more job oversight:

Don't get in trouble for slipping your cat food into the petty cash fund! c1.staticflickr.com

Being a bean-counter is one of the worst parts of running a business. Keeping track of every little expense can be tedious, even for the most Type-A organized people. Petty cash privileges are implied in a lot of jobs. If you've been doing a decent job of gathering up your expenses and filing them without getting them bounced back for careful review, that's a sign that you're being trusted to do the right thing. If, however, your boss makes a change in tone and requests receipts and lengthy-explanations for every penny spent, you may be getting a signal that things aren't as copacetic as they used to be. Be sure to keep all of your receipts in case accounting suddenly becomes more intense.

It's important to listen to your intuition. If you begin to have doubts about your standing in the workplace, don't push those feelings aside. Don't panic—but pay attention to warning signs and address them as they arise rather than let them pile up.