According to Apple Support:
If you use a screen saver, it starts automatically after your computer
is inactive for a time. You can also choose a shortcut so your
screen saver starts when you move the pointer to a corner of the
Apple menu >
System Preferences, click
Desktop & Screen Saver, and then click
Screen Saver pane of
Desktop & Screen Saver preferences
Hot Corners. Choose
Start Screen Saver or
Disable Screen Saver from the pop-up menu for a corner you want to use; and
then click OK. To use the Command (⌘), Control
(^), Option (⌥), or Shift (⇧) key
together with a hot corner, press the key while the pop-up menu is
open. For example, if you press the Shift key, the Start Screen Saver
command changes to ⇧-Start Screen Saver.
To deactivate the screen saver and return to the desktop, press any
key or move the mouse.
Additional screensaver configuration options for Lion are listed in this Apple Support KB.
This issue could definitely be a bug, but you could also check your screensaver configuration to determine whether you've turned on hot corners - it's possible that your mouse pointer may have triggered the screensaver. You can also choose to disable screensavers completely to avoid this issue.