I've been seeing a lot of confusion recently about why certain replays don't work, giving a sync error message and displaying odd unit behavior. This is the explanation:
Normally, only replays recorded using the same version of Company of Heroes are visible in the replay list. This is because replays aren't stored as videos of what went on, instead they're just a series of player instructions. When you view a replay, your computer is re-enacting the battle using the commands stored in the replay file. When Company of Heroes is updated, things like unit stats and behaviors can change. When the re-enactment uses these updated values instead of the values that the replay was originally made from, the re-enactment can go differently, changing the course of the game and making the replay useless. Generally, this is manifested through the Sync Error, and lots of weird unit behaviour.
Due to a bug with the most recent version, replays recorded using the previous version continue to be shown in the replay browser. However, these replays are incompatible and will produce this sync error message. So if you see this error, double check the version of the file – all 2.501 replays will not work in 2.502. Sadly, this means all of the 2.501 replay packs solicited in the recent contest cannot be viewed.
Fritz Frisbee reminds me that synch errors will also show up in replays where a player has dropped.
