I've contacted support directly about this issue, but it's hard to nail it down, and I'm wondering if other accounts are having the same issue we are.
In brief, users sometimes find themselves logged in as a different user, even though they have entered their own correct PIN. We have only noticed this behavior when logging in to Lightspeed on computers which have just woken from Sleep mode. They are able to perform any actions that don't require them to log in again (in our system, that's anything besides a new sale or new workorder). So, as an example, one employee came in to work in the morning, logged in, and started receiving purchase orders as a completely different user.
You can imagine the problems this could cause, even absent any ill intent whatsoever.