Clients are often fanatical about deadlines. They need the paper checked by 6pm, you need to show up 20 minutes early to the test site, you should be the first person there before conducting training, but when it's time to pay you for your work, here come the excuses. It's payroll's fault. Our funding didn't come in. The manager who has to sign off is on a business trip. We forgot. You forgot?!?!
One of the things that eases the burden of unsteady income is having a healthy accounts receivable balance. Yes, it's just another thing to keep track of but this is your money we're talking about. It's worth tracking. Generally, clients that you have a strong personal relationship with will nudge the appropriate people to get you paid on time but you don't always get to work with people you know well and like.
I strongly recommend that people who are considering freelancing full time try to keep a "cushion" of two months of your average salary as your bank account balance. Account receivable are good in the sense that you know that money will be arriving eventually but when you have to pay the bills and buy groceries, that cushion will come in handy when business is slow.