When you have memberships being edited in multiple tabs the information from one will replace another.
To replicate (this works on the current demo)
- Access Memberships dashboard
- For one record (A) right click 'edit' and open into a new tab
- Then for another record (B) right click 'edit' and open into a new tab
- Go to the tab for record A, make no changes and press 'Save'
After doing the above you will see all of the details (Level, Date, Source, Since, Start/End Dates) for record A will be saved to B along with the success message displaying you have successfully updated the record for the name of record B.
I'm assuming this is some kind of session error but it's a major error that we've just discovered on a client's setup that has caused membership records to now have the wrong levels set.