I like to keep a copy of the OS X installer app on an external drive, so that I can reinstall or create a boot disk without having to download the whole thing again.

After doing this, I’ve discovered a non-standard behaviour which you might want to note.

If you decide to redownload the app, it doesn’t get saved to the Applications folder as before, but instead REPLACES the copy on your external drive.

This makes it hard to keep archive copies of older OS versions. You’ll need to zip them (or rename them?) or store them on a volume that can’t be accessed while the app store is downloading.

Source: Mac OSX Hints