Here's an email and response from WUA (Rick Roder) made 1/03:
Questions from Steve Freix :
Extra ball appears while play is occurring.
Pitch is low, deflected by catcher, and hits bottom of ball bag knocking extra ball onto field. Original ball deflects about 6 ft. away while ball from bag drops straight downward. Catcher, who is unaware of what occurred, fields the wrong ball and makes a play retiring an advancing runner. What do you do?
A similar situation could occur where an outfielder, diving for a ball along the line misses the ball which rolls to the fence. Fan of the defense drops ball onto field much closer to fielder which he unknowingly plays into the infield. Is this fan interference, or would a fan have to physically contact a ball or player on the field--inclusive of throwing items at a fielder, I assume?
What if this ball was not dropped by a spectator but was an errant warm-up throw out of either bullpen? Would this be interference per 3.15? If so, it's unintentional which says play should continue without umpire judgment coming into play---correct?
Importantly, what if you judged that the fielder had opportunity to know which ball was which and intentionally chose to play the incorrect ball?
[AND HERE'S THE RESPONSE]
The response to all these scenarios is the same. The original ball is in play unless an umpire is unable to determine which ball is the correct ball. At that point he calls time and decides what would have happened without the mixup. He then awards bases, calls outs, whatever he decides needs to be done to rectify the situation.
[my emphasis]
Thanks for your questions!
World Umpires Association
Just thought I'd share this.
Call 'em out or award bases, whatever you feel appropriate to rectify situation
caused by an extra ball----
but don't do anything to kill the ball or stop the action until the extra ball on the field actually interferes with play.
Just my opinion,
Freix