Ask HN: What deprecating guides are there to minimise problems?

What problems have you encountered when not deprecating or deprecating features after updates? Especially for web applications and databases?

What guides are there for deprecation? I found Java: http://download.oracle.com/javase/1.5.0/docs/guide/javadoc/deprecation/deprecation.html

The problems I can think:

+ a 404 when old links point to old locations.

+ Form pages submit old form data instead of new form data.

+ Submitted data tries to go to a database table that doesn't exist.

What's the best way to determine a minimum time for the deprecation? What about public APIs?