Which underhanded practices have you seen companies employ to stop their employees from resigning.

The title says it all. For context, I went for an interview with a Multi National Company in India when I was a student. I remember the company asking us to sign a notarized paper (stamp paper it's called here) saying we would work for a minimum of two years and not leave otherwise we would have to compensate the company by paying three month's wages. What other deplorable acts do you know of that companies use to make their employees stop leaving