My wife and I ended up moving (within California) on short notice that overlapped my wife's unrelated trip out of the country. Despite very clear documentation on the USPS website about the documentation required for me to verify my wife's identity (and my relationship to her) in order to complete a Change of Address order on her behalf, the USPS employee immediately and aggressively accused me of attempting identity theft. No amount of showing him the USPS documentation about how to verify a spouse's identity would convince him.
At the second USPS office I tried, the guy was very nice, but also said that due to a high rate of identity theft they are refusing to do anything without the person there.
Luckily we only had a short interval where mail wasn't being forwarded before my wife returned and verified her identity in person, and probably didn't miss any important mail.