You know when you see a job posting and it lists an application deadline? And then you figure you have until that deadline to apply?
Well, you might not.
Here's why: A lot of job sites require the employer to list a deadline or closing date when submitting a post. There's often no option for "we're looking at candidates on a rolling basis." So employers are forced to pick a date, even if it doesn't reflect how they're actually handling the search.
This is a bad system, because it forces employers to list information that might not be true and that will mislead candidates. On the candidate side, you see that date and think "great, I have four weeks to submit my application." But if the employer is actually looking at applications as they come in and talking to good candidates on a rolling basis, when you apply in four weeks, they may have already assembled a group of finalists they're excited about, and thus the bar is going to be a lot higher for adding someone else in last-minute. Or they could be poised to make someone an offer, or the job could even have been filled already.
Of course, like everything in hiring, this is not universal. Different people handle things differently. But from the outside, it's hard to know. (Personally, I will take a fantastic candidate at any time, even if the deadline has passed -- I'm not going to turn away a great candidate because of an arbitrary deadline. But I also evaluate applications on a rolling basis, and if I find someone great before the deadline is up, I'm not going to risk losing her to another offer while I wait for the clock to finish ticking.)
So if you see a job you want to apply for, apply now, no matter what the application deadline says.