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Employers can—and sometimes do—renege on a job offer.

“American employees are employed at will,” says Lewis Maltby, president of the National Workrights Institute. Which means the boss can fire you for no reason, and can withdraw an offer at any time, except for reasons related to race, gender, age, or other type of discrimination.

In fact, it’s not unusual for organizations to make their offers conditional on any number of factors. Those could include a criminal or credit check, substance test, and reference checks, among other things, says Christine Walters, a human resources and employment law consultant with FiveL Company.

The job may also hinge on securing funding from investors or achieving some other milestone. “Government contractors often make them conditioned upon winning a particular contract on which they are bidding,” says Walters. “Nonprofits may make it conditioned upon obtaining a particular grant for which they have applied.”

Read next: Is It Wrong to Back Out of a Job Offer That I've Already Accepted?

Where it gets tricky is when an employer withdraws an offer late in the game—namely after the candidate has quit his or her old job or turned down other offers. Even worse, there are (rare) cases where a person quits a job and relocates only to find no job waiting on the other end.

Fortunately, “employers don’t change their minds very often,” says Maltby. In most cases, they are just as eager as you are to close the deal.

Even so, don’t consider the job officially yours until you see it in writing, even if the person offering the job runs the company.

In one case, says Walters, the CEO of a company had offered the candidate a job. “When he arrived for his first day of work, he was told the CEO had been fired, no one was aware of the offer, and the offer was not valid because company policy required every offer to go through HR before being extended; the CEO did not follow the company policy.”

Read next: How to Make the Most of Two Competing Job Offers

In this case, the candidate sued on the basis of detrimental reliance—relying on the offer to his detriment—and won. While he did not have an offer in writing, he did have the word of someone who, in this case, had authority to speak on behalf of the company.

Though the law sided with the candidate in this case, better to be on the safe side and insist on an offer in writing before you quit your job, turn down other offers, or make relocation plans. Even better, says Maltby, follow up with a phone call to make sure there are no loose ends or contingencies.