Sadly, for "paid in advance" deals, that is more common than not. It's particularly bad with individuals, but I've even heard (and experienced) this with larger companies.

After 6 years in the industry, we simply never pay in advance any more, because we've been burned with late deliveries and bad work too many times.

If you have enough of a reputation for being stand up, people will generally agree to be paid after delivery. If not, suggest that you work through guru.com or rentacoder.com, both of which will escrow and arbitrate disputes.