Holding a card by accident
According to the ACC's tournament rule 7.3, on reneging:


  Any cards played after the renege occurs or the pegging of a "go" shall be retrieved by both players, and any points pegged (or scored) are retracted. The non-offending player receives two points for each card that singly could have been played (judges confirm number of cards). The nonoffending player decides if the reneged card(s) will be dead or played. Play continues, and all cards in both hands and crib are counted. 


In your example, the play would be brought back to the point of the incorrect go call, the non-offending team would peg two points, and then decide whether or not the remaining six would get to be played.  The scoring of the hands would be unaffected.