After sitting for a wayward account, I backed out to my own account. The count on messages was extremely high in the bar at the bottom of my screen, while mine shouldn't be, indicating that the state of the screen did not refresh. I clicked on messages and was able to view the actual private messages of the account I was sitting. Logging out fixed the issue, but I should never be able to view somebody else's private messages!
A long time ago (2 years or so) I experienced exactly the same after sitting someone, but it was only for once and it never happened again, so I thought it was a one-time glitch....
Apparently it isn't
thank you for your report! Which browser do you use?
This also happened to me previous round on COM2 using the android mobile app. I think it had something to do with other issues the app has, like sometimes when the app lost focus (when minimizing it) the top bar displaying money, prestige, gold was switching from the stats of the account I was sitting to my own. Needless to say I wouldn't be able to switch trains anymore and had to restart the app since even closing the sitting screen wouldn't work anymore.
Browser: Firefox 77.0.1 64 bit