This should be built into your hourly rate. An hourly rate needs to not just cover billable hours (aka time programming), but also bug fixes, customer support, etc.

This is the reason (among others, such as lack of benefits) that hourly rates as a consultant/freelancer are so much higher than that of an employee.

That being said, if you do fixes like this for free, they'll bug you (hey, it's free -- why not?) rather than spending half a second figuring it out themselves.