Mark Pilgrim: " It checks for June 31st. I swear to God it does. One day I was writing test cases and Sam was writing code to pass them, and when he saw that test case fail he almost reached through his cable modem and strangled me. He almost removed the test case out of spite. He gave in and coded it anyway, and checked it in, and we deployed, and three days later I got a bug report from someone who couldn’t figure out why his feed wasn’t validating. And I couldn’t figure it out either, until he mentioned that it only seemed to choke on the date for one specific entry, and I looked at it one more time and I swear to God it said '2006-06-31'"

There are no exceptions to Postel's Law on the Internet. Upstream servers will break downstream clients irrespective of what we regard as the finished downstream article. This is normal and to be expected.

1: Every downstream you visit that refuses to handle the law will be dead, or dying.

2: Everyone is downstream.

In particular I want to dispose of the idea that negotiated agreements are a let. No SLA gets you out of Postel's Law. Just because we have recently starting calling these contracts "APIs" makes no matter. Postel's Law holds, I think, the supreme position among Internet principles. If your client is found to be against Postel's Law I can give you no hope.