Some users prefer to have their applications open whatever documents or windows they left open when they last closed that application. Users who want to pick up where they left off in this manner will find that setting preferences to enable this behavior changes when they migrate from OS X Lion to OS X Mountain Lion.
First let's see how you do this. Then we'll discuss whether it's wise.
OS X Lion: Restore Windows when quitting and re-opening apps
Open System Preferences in OS X Lion.
Click the Show All button.
Click the General icon.
Check the box labeled Restore Windows when quitting and re-opening apps if you want to re-open all windows or documents when you re-launch applications.
OS X Mountain Lion: Close Windows when quitting an application
Re-opening documents or windows may not be a good idea
When you takes advantage of this convenience feature, you run the risk of re-visiting a malicious hyperlink, page, or document. Users can end up in a closed loop of mischief of this kind:
- You open a browser, visit a web page
- A script on that page executes and some badness wrecks havoc on your Mac
- You quit the browser
- You discover and undo (remediate) the harm
- You re-launch the browser (a.k.a., lather-rinse-repeat)
By enabling this preference, you're permitting not only your browser but all applications to re-open the last or recent document or window. If you use MS Office, for example, you may re-open a Word document containing a nasty macro that arrived as an attachment, or a malicious PDF or Flash file with an Adobe product.
Before you enable this behavior, ask whether you want to open whatever you last opened, a favorite site, or (the safest course) start anew. Then explore whether you can accomplish what you want without the broad and potentially nefarious "allow any" preference. It's unfortunate that convenience features can put your Mac at risk, but now that you're aware, choose wisely.
(Note: Certain browsers have similar options or addons. Firefox, for example, has Show my windows and tabs from last time under General -> Home Page. Safari has SafariRestore. Explore other applications you use frequently understand how they behave when opened.)