The Xbox 360 version of Left 4 Dead received a bit of spit and polish with a recent patch, with the happy result of restoring achievements to those who lost their progress thanks to add-on The Sacrifice.
Valve has said progress towards achievement tallies made before players updated to The Sacrifice will be added on to that made since.
The fix should resolve the problems of the "majority of players", although some will apparently just have to lump it.
The rest of the patch notes are below, courtesy of the Left 4 Dead Blog.
• In Versus mode Tanks will spawn in stasis until a human player takes over. Tanks in stasis are invisible and invulnerable, and won't move or attack.
• Tank speed is no longer reduced when on fire.
• Allow players on the Infected team to manually spawn during finales.
• Added a vote option to toggle All-Talk.
• Special Infected that causes a Survivor to stumble into a "ledge hang" situation will receive 50 points.
• Bullets that first pass through an infected character before striking a Survivor are now ignored as friendly fire.
• In Versus mode car alarms will now trigger anytime a Special Infected pulls or stumbles a Survivor into an alarm car.
• Survivor Bots no longer try to melee exploding barrels out of their way.
• Fixed the Sacrifizzle achievement,.