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Users who have previously downloaded the free 30-day iWork trial and kept it on their systems found themselves able to update to the latest version of Apple’s productivity suite for free yesterday due to what is apparently a bug with the way the store finds copies of the software purchased through other sources.

Since the Mac App Store now detects boxed copies of iWork and allows them to be updated to the Mac App Store version, it seems the trial versions of Pages, Numbers, and Keynote are perceived by the App Store as being a full purchased copy. This allows users to install yesterday’s updated apps without having to pay anything.

While the suite is free with the purchase of a new Mac, users on older machines who have never purchased any version of iWork in the past must pay the full price for each app. This bug allows those users to circumvent that process.

It’s unlikely that Apple can even fix the issue, since the trial versions of old iWork apps are identical to the retail copies (but require a serial number to activate). Since the apps use the same identifier whether activated or not, the App Store has no way to determine if the trial has been activated.

Update: Aperture users are reporting that the trial version of that app also allows a full upgrade for free.