That's nasty - it should never do that. Normally it works in the other direction, upgrading you to Debug Player 10.1 if you don't have it already.
A few Q's:
Are you on Mac or Win?
What browser did you see this in?
As a workaround, you can find the Debug Player 10.1 installers in your root install of FB 4.0.1, in the Players folder. You can re-run them quickly from there, without needing to download them.
Thanks for the quick response, Chris.
I am using Windows 7 (64-bit) and I have seen this in Firefox 3.6.6.
I will re-install Version 10.1
I need to take this back.
What happened was that I was by mistake installing FB 4.0.0 Plug-in. The FB 4.0.0 Plugin downgraded the Flash Player in Firefox.
The FB 4.0.1 Plugin does not downgrade Flash Player Plugin.