Back in the old days, if a Macintosh computer crashed, certain files would be damaged, and that would be it. All of the data on the drive would be lost, which was a serious setback if one had not diligently backed up files.
Total data loss is still a risk today, even though the latest operating system, Mac OS X, rarely crashes.
The biggest causes are defective or low-quality hard drives. The tried-and-true solution is still to back up data regularly to media that can be stored offsite from the main computer.
I find, based on my experience working for a small publishing company, that the most common data losses come from people who are in an editing frenzy and not from hard-drive crashes.
Here are common examples: Employee No 1 opens proposal A, and edits it for another client. Six months later, he wants to know if I have a copy of the original proposal.
Employee No 2 deletes all her e-mails regularly after two months, on the assumption that I keep copies on the server. This is fine - until she requires a year-old e-mail that ended up being a contract for a price, according to the customer.