TRANS FAILSAFE PROGRAM. May this never occur again?

[goldenbird]

The same day I purchased my car a week ago. I got the TRANS FAILSAFE PROGRAM. I am afraid it is a sign of bad tranny. The mechanic checked the error code(wrong gear ratio when upshifting from gear 3 to 4). Bat voltage was 13,7v.

The mechanic cooled me down as a slipping problem between gear 3 and 4 is rare and the chance for a big gear size to be eaten up is not highly probable. He thing there is a 75% chance that this might never happened again. What do you think? is there anybody that faced this just once with no second occurrence? I am afraid to flush the tranny oil because there is a lot bargain about such a step