autofs appears to be started before it is truly functional
Binary package hint: autofs
Description: Ubuntu 10.04.2 LTS
Brief summary: autofs, in upstart terms, becomes "started" before it is actually ready. There seems to be a brief window immediately after "started" where if you run something expecting to be able to read or write to an automounted directory it will either fail (on a read or a write to a subdir of the mount) or write to the empty mountpoint (on a write to the root of the mount). Even though `status autofs` is "started/running" the mounts aren't ready.
If you sleep for a few seconds in the post-start script it covers up for the bug in most cases (probably) but the exact number may vary by system.