I tend to set long movement paths for my units in the mid game. Typically long enough that it takes 2-3 turns for a unit to complete the path.
What I've noticed is that when I start a turn, if I move other units so that they end up sitting on an existing movement path when I get around to moving those units (via the move all button) I get an Out of Sync error when the units get to the point of crossing my other units. This causes a full refresh of the browser.
To repeat:
1) Set a long movement path for a unit/stack that takes more than 1 turn to complete.
2) On the next turn, move another unit/stack so that it sits in the middle of the uncompleted movement path.
3) Move the original stack via the move button.
4) A resync will happen as the stack tries to cross the new stack.
This is most noticeable when setting a path into a city and the arriving stack forces a refresh when crossing units that are in the city (ie stack arrives from south side of city and causes a refresh when moving to the north side across units sitting in the south part of the city).
KGB