The unhelpful error message when 4.1 client connects to a 4.1.1 server:
Attachment:
4-1-1-error.jpg
That's because zpak001_assets takes precedence over zpak000_assets, and if you don't have one, you're assumed to not have the map. Even if you do (most of the maps aren't updated).
EDIT: Some maps (Uptown for example) give a different message: CM_InlineModel: bad number. The same thing can happen if you replay a 4.1.1 demo on 4.1 setup.