Any changes a Web Designer+ makes to the shared version of the MySite pages will
be seen by all. They need to make sure that they are in their Personal View (it
will say "Modify My Page" in the top right corner of the page) rather than the
Shared View (it will say "Modify Shared Page" in the top right corner of the
page) of their My Site when they make changes. Otherwise, they are changing the
view of that page for everyone in your organization that has not themselves
personalized their view.

It is also possible that they customized (unghosted) the MySite page. In this
case you can download, install, and use the Ghost Hunter Web Part from the
WebPartToolKit found  on http://bluedoglimited.com

To prevent  this from happening  in the future you need to educate your Web
Designers and Administrators. I would also block FrontPage from all production
Site Definitions. You can do this by adding the DisableWebDesignFeatures
attribute to the Project element in the ONET.XML file in each Site Definition on
every Web Server. See the SDK for details:
http://tinyurl.com/5m9tz

I wasn't aware that Web Designers
could hose MySite for everyone like that. Is there a way to reset  it
back to OOB?

Only manually. Just reverse the actions of the Web Designer.

All other solutions (restoring from back, delete and recreate, etc.) are far
more complicated.