What happens is the info is still up when the screen moves, like when you're jumping to the next active unit, or exiting the city screen. So it sticks. If you 'mouse-off' before the screen jumps, it won't stick. However, due to lag and such this is nearly impossible to avoid. Those with older machines will feel it worse unfortunately.
This has been around ever since the PLE mod was added. I believe it was fixed temporarily, but that broke something else, so we had to live with it. Most of us, I'm sure, think this is a bearable side-effect, considering the benefits of the PLE mod.
To 'un-stick' the info, just mouseover whaever type of info has 'stuck': units or PLE tooltips, usually.
KEY: PLE= plot list enhancement, an enhanced user interface for manipulating multiple units on a single plot or tile.