Posting to say also say I was logged into somebody else's account when first loading MangaDex today.
@ixlone although I was "not actually on their account", I certainly had access to see things I shouldn't have been able to see:
Site-wide settings like theme, language settings,
H(
!?) were [temporarily] changed -- along with being able to see their follows, reading history, etc.
(I think their "Last online:" activity timer also updated too, but I can't be certain of this one)
I checked the user's public settings afterward and their MDList isn't public, so I don't think I should've had access to even view a few items of what I listed even if only temporarily.