Can an employee make a server pay for a customer walk-out?

In CA, if are an hourly restaurant worker and a customer does a "dine and dash," your employer generally cannot deduct money from your paycheck. The exceptions to this are:

Your "dishonest or willful act" caused the loss; or 
Your "gross negligence" caused the loss. 

Ordinary mistakes do not fall into either of these categories. Therefore, since "dine and dashes" are usually beyond the control of employees or are usually caused by ordinary mistakes, they do not allow an employer to make a paycheck deduction.

The employer can only deduct the money if there is an agreement to do so: preferrably, a written agreement, signed by the employee, acknowledging the employee's liability for any short or missing payments. If there is such an agreement, you have to pay; if not, he legally cannot take it out of your check, and if he does, you could complain to the state department of labor and/or sue (such as in small claims court, as your own attorney or "pro se") for the money.
Of course, the employer could make signing such an agreement a condition of employment--i.e. he may fire anyone who won't sign. And if there is no written agreement, but he asks you to pay for a given dine and dash or the like that he thingks is your fault, he could fire you for that: for costing him (he believes) money, without compensating or reimbursing him. So while he can't simply take the money without your agreement, he can make agreeing a condition of continuing to work there. 
You would be well advised, if at all possible, to seek other employment: this is not someone you want to keep working for.