All My activities show for other person name
The Problem

When you are logged in to CiviCRM, ALL public-facing pages (event registration, contributions, etc.) will assume that you're the logged-in person.  There's a warning on the page with a link to let you register/contribute as someone else (see screenshot below).

Preventing this

All registrations/contributions should be added via the back-office screens, not public screens.  If you need a public screen, use the link circled below.

Furthermore, the excellent No Overwrite extension should be installed wherever a) it's possible that someone might overwrite another record, and b) you don't need to permit visitors to overwrite their name during registration/contribution.

Fixing After it's Happened

This must be fixed by hand.


Figure out all the information on the shared contact that belongs to the new person.  You can figure this out by looking at the public-facing page.  Any fields on that page are most likely overwritten.  
Put the fields (name, etc.) back the way they were before the bad transaction.  If you're not sure, you may want to load a backup of your database on to a test server to compare side-by-side.
Create a new record (if necessary) for the new contact and populate it with their info.
Additionally, this record will have one or more contribution/membership/participant/activity records associated with the transaction.
If you have back-end access to the database, change the contribution/membership/participant/activity records so the contact_id is changed to the new contact.  
If you don't have back-end access, you'll need to delete the transaction and do it again (don't forget to refund the money before charging again, if applicable).
If the contact was supposed to be added to a group based on a profile in the page, don't forget to add them (and potentially remove it from the mixed contact).