I got an email this week from a vendor whose product I reviewed a while ago. I'd mentioned in the review that one of the features wasn't covered by the manual, and as they were in the process of revising the documentation, he was wondering which feature that was.
It's increasingly common for features of a device or application to be omitted from the manual. The reason's obvious: a simple change that takes ten minutes of developer time might take an hour of a documenter's time - ensuring that the change is reflected in all the documentation, perhaps taking new screenshots of the relevant bits of the GUI, and so on. So for the first release, manual and application perform in sweet harmony; but from version 1.1 onwards, the tech writers are running to keep up.
Just because documentation takes a long time to produce, though, this doesn't justify allowing it not to keep up with the product. Even simple changes can lead to hours of frustration on the part of the user.
Worse still, because the effort required to documentation new features often exceeds the effort expended in adding those features, it's all to common to find that the stuff that does make it into the manual gets just a cursory mention, with only scant coverage and much of the functionality left to the imagination. For instance, take the "Enable Stealth Mode" button on a popular firewall. The total coverage in the manual of this feature was: "Enable this to set the firewall to operate in stealth mode". Did it give any clue as to what Stealth Mode actually was? No. Thankfully they've seen the light, and in the new version of the manual things are much better: "If enabled, the router will not respond to port scans from the WAN, thus making it less susceptible to discovery and attacks".
The vendors are, of course, in a Catch-22. The users are asking for fixes and new features, and the vendors have a choice of (a) be quick to market but without proper documentation; or (b) hold the release back until the docs are finished, thus being slow to market but accurate in the manual. So there's no right answer - which means that no matter what approach they take, the vendors are fighting a losing battle.