was trying to sort population and move a peasant to dust production and it suddently disappeared and my borough prodution went from 23 to infinity, while my population went from 6 to 5...

WTF?

Did this happen in multiplayer?

We'll need a little more information on this:

Did this happen during a normal turn, or was the turn already processing? Do you have a save game where it happened, and information about which city to check in that save?

I actually encounter this issue regularily in multiplayer games. It always happens when I get an additional pop through villages (pacification or construction complete). If I switch around pops on that very turn (sometimes I need to do it a while), the pop will disappear. However in the next turn, it reappears magically, and after that everything works fine.