When a military site is attacked it may occur that a soldier that was previously engaged in a fight on the flag before the site enters the building and another one steps out. The second does not proceed to the flag but stays in the doorframe and starts fighting without opposing enemy on the node. The enemy is waiting at the flag. Thus no soldier can enter or leave the military site. Thus the fight cannot be completed.
Destroying the building freezes the animation.
A similar behaviour can be observed if an enemy site is attacked an soldiers leave the building to engage the enemy. Sometimes a soldier stays in the door frame until a returning soldier sweeps it up after the battle is finished (this would probably block if the battle is not won).
Revision 5778 but also observed in previous builds (~5600)
See savegame at (34 52)
Might possibly be related to bug #612348 soldiers do not finish their battle
we are in dire need of a replay. Please check in your replays folder if you can find the replay that lead to this bug; the savegames did not help in fixing it. If you can provide one, we might get closer to finally fix this.