One other possibly sneaky area is when companies shift to unaccrued or "unlimited" vacation and the company puts a culture in place that discourages using that vacation. I'm fortunate to currently work in a place where the unavoidable crunch times are followed by vacation my manager almost forced me to plan to avoid burnout.
from http://www.dir.ca.gov/dlse/faq_overtime.htm
If you are paid a salary, the regular rate is determined as follows:
Multiply the monthly remuneration by 12 (months) to get the annual salary. Divide the annual salary by 52 (weeks) to get the weekly salary. Divide the weekly salary by the number of legal maximum regular hours (40) to get the regular hourly rate.
The idea being, I think, that working 120 hours deserves additional compensation, whether you have a week to recover or not, and companies can't game overtime by giving you vacation days to recover. They would be better off just having you work regularly.