You've probably still got an old plugin file.
Do a complete clean install:
Spotlight search “Flash Player.plugin” (Search “This Mac” and choose "Show All in Finder")
Right click the Plugin and select “Open Enclosing Folder” Trash the Flash Player.plugin and flashplayer.xpt files.
Go to: [user]/Library/Preferences.
Note: You'll need to hold the "Option" key when you click "Go" from the Finder menu to access the [user] Library:
Without the "Option" key
Trash the Macromedia folder there.
Go to: Mac HD/Library/Application Support. Trash the Macromedia folder there.
Go to: Mac HD/Library/Preference Panes (not Preferences, but Preference Panes). Trash the Flash Player.prefpane file there.
Empty the trash.
Download the Flash Player (For Mac OS X) again if necessary.
Quit Safari and go to your downloads folder. Double click the DMG to “mount it”.
It will appear on your desktop and a small window will open.
Double click the installer. If you see a warning, click “Open” to let it run.
Click “I have read and agree… etc.” and click “Install”. Enter your password when asked.
You may see the following: click “Force Close All” if you do, and then click “Retry”.
The installer will run.
Choose your update preference, and click “Done”.
Open Safari again and click Safari>Preferences (Cmd+,)
Go to the Security tab. Make sure “Allow plug-ins” is checked.