Forge Network Handshake Out of Sync (HandshakeAck Cannot Be Cast to RegistryData)

What it looks like in the log

java.lang.ClassCastException: net.minecraftforge.fml.common.network.handshake.FMLHandshakeMessage$HandshakeAck cannot be cast to net.minecraftforge.fm

How to fix it

1. Completely close and restart your Minecraft client before reconnecting, because Forge 1.12.2 does not cleanly reset its registry handshake state after a failed login attempt or singleplayer session.
2. Open your client logs/debug.log (or latest.log) and look immediately before or after the ClassCastException for a root-cause error such as IllegalStateException: Could not determine owner for the override, which names the exact mod ID breaking registry sync.
3. Ensure your client mod list matches the server modpack; if the error started right after adding extra client mods, remove those added mods and test connecting again.
4. Update both the server and client to the latest recommended Forge 1.12.2 version (14.23.5.2859), as older 14.23.x builds contain bugs in registry override handling during handshakes.
5. If a mod was recently removed from the server and logs mention an orphaned block override, back up the server world and either re-add the removed mod or remove the stale registry override entry from the world's level.dat using NBTExplorer.

Check your own log