If you manually add/remove contacts to a Smart Group will they remain in the group when the group updates?
As Sanjay said, yep it'll work. In practice we've found it causes trouble, though. In our experience it confuses non-technical users, who will then merrily start adding/removing people from smart groups directly (rather than changing the underlying criteria that the smart group was built on).
It also causes odd UI issues - I've definitely seen pages where manually-added people aren't included in counts, though this is probably a bug.
There's a good reason that smart groups have this feature - if you use a smart group as a mailing group, then you'll often want to allow recipients to remove themselves from the group (even when they still satisfy the smart group criteria). It'd be nice to be able to be able to track these, and you can in theory since adding and removing contacts leaves a trail.