I started the game offline once, and I received a warning about that. I also noticed that achievements were not saved for that session.
What are all the game/metagaming features that don't work while offline?
Starcraft 2: What features are disabled/different while offline
starcraft-2
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Best Answer
Any achievements you complete (e.g. complete all mission objectives) will not be saved. The mission will still be complete, but you need an active connection to battle.net to activate the achievement -- meaning you need to do it again if you lose your connection.
Additionally, while SC2 uses cloud storage to save your saved games / campaign data, this cannot occur while offline. After regaining connection to battle.net, your saved games / campaign data should continue to update.
All Challenge maps are available.
Custom games, vs. AI only, are available. (Actually, not 100% sure about this. I think I played a custom game vs. AI while offline)
Edit: Map Editor and testing maps made in the editor also do not require an internet connection (the latter not even requiring you to log in)