Just a heads up that after todays maintenance thingy I refreshed the page and seemed to have logged on as some other user. After refreshing again I was back on my actual account.
It's just a cache issue, you do not have access to anyone else's account because as soon as you click anything that requires a reload/redirect etc, the cache is refreshed and you will see the normal account screens
The short explanation here is that nobody has actually "logged on" to anyone else's account, you were simply served some html that was meant for someone else due to things being a bit fucky at the moment. The problem is essentially just visual. At worst someone's going to see some follows that belong to someone else.