This has burned me more times than I’d like to admit. I have a macbook for work and activated iMessages for my personal account. One day I accidentally posted something in a chat room meant for someone I was chatting with in iMessages. It was kind of personal but thankfully vague enough that I could brush it off. I also was on some screensharing sessions when I switched to iMessages which briefly showed personal conversations. After a few times of that happening I disabled my iMessages account for on that machine and made it a personal rule to never mix business and personal activity on any company-issued device, which has served me well over the years.
Somehow they made it impossible to specify or indicate the account in the URL to squash any uncertainty, you just have to hope the cookies are right.
So a ".../mail/u/0/" means the first Google account you're logged into, ".../mail/u/1/" means the second, etc. But if you did it in a different order on a different machine (or even browser), it'll redirect to a completely different one.
I would love to have been a fly on the wall in the meeting where it was decided that was a good idea.
I know my willingness to explore UIs went down a lot when
1) All the buttons became icons instead of text that clearly explained what was going to happen and
2) My phone became the place where all of my social media accounts were always logged in and all my internet activity was centralized. It became easy to accidentally share between different worlds.