Scoring Question : Is it an error

To the best of my knowledge:

If, in the official scorers' opinion, the foul ball should have been caught it is an error wheather the hitter follows w/ a base hit or not.

The dropped foul ball prolonged the at bat.

Refer to MLB official rules/official scoring. The rules should be the same across the board in this case, I would think.

Is there any other instance where a defender can make an error while in pursuit of a foul ball? I think it's interesting that an error can be charged when a ball is in foul territory.

Just think of it as if you were a pitcher. Say there's 2 outs and no one on and the catcher drops a foul pop up. The batter ends up hitting a HR. It's not the pitchers fault and the run is rightfully unearned. It's still an error even if hte batter strikes out. Just like if there were 2 outs and no one on and the shortstop made an error but the next guy struck out or hit a homerun. It gave the batter a chance he didn't deserve.

If a foul ball is hit in front of 3rd base and it is not caught, is that an error also?

Yes. Error. If the ball should have been caught foul or fair doesn't matter.

Doesn't matter where the ball is. If a player clanks a fly ball anywhere on the field and if in the official scorers judgement the ball could have been caught w/ ordinary effort it is an error.

If it happens to be foul, it prolongs the at bat. Pitcher is therefore penalized so to speak. (as mentioned in other posts above)

Wow! Thank you very much for setting me straight on that!