That's true, this can happen. At Mixpanel, we proactively true people up throughout the year, each year. It's not that hard of a pill to swallow when you weigh the decision of that person leaving vs paying them $10-20K more.
If you're in SF and your rent is rising by 10% p/a then your salary needs to be keeping track unless you already have FU money.
Or you work a couple of years to get a little vested, then jump ship using your unvested equity as a bargaining chip in your next negotiation "I'd be leaving so much on the table..."
I guess that there's a reason that the average tenure is ~2 years