Last month, Google updated Chrome to version 20. As part of the upgrade, the browser's built-in Flash plug-in was moved from the ancient Netscape plug-in API (NPAPI) to Google's newer Pepper API (PPAPI). Unfortunately, by default, Chrome doesn't disable the legacy plug-in and leaves it running side-by-side with the PPAPI one. Fortunately, it's simple to switch the old one off.
As Locker Gnome advises, just type "about:plugins" into a Chrome tab and you'll be directed to the browser's plug-in configuration page. "Shockwave Flash" is what you're looking for — the screenshot here shows what you should see. If you can't see the extended options, make sure to hit the "Details" box in the top-right corner to expand each item.
The API used by each plug-in is listed next to the word "Type". The new Flash will specify "PPAPI (out-of-process)" while the old one will have "NPAPI". All you have to do is click the "Disable" link next to the NPAPI plug-ins to deactivate them. If you don't see a PPAPI plug-in, chances are you haven't updated to the latest Chrome.
Alternatively, if you've found Chrome's Flash capabilities have been letting you down since you updated, you can disable the new PPAPI plug-in.
How to fix Shockwave crashes in Chrome [Locker Gnome]