The opposing member inadvertently plays two card that are stuck together and it is discovered later
Assuming you are playing rubber bridge, the Laws of Rubber Bridge apply.
If the irregularity is discovered part way through the hand, was a misplay by a Defender, and during discovery of the misplayed card it was ever placed such that Offender's Partner could have seen the card then it becomes an exposed card subject to the Penalty Card
rules.
Additionally, if the irregularity is discovered any time before the play is complete and the cards collected for shuffling, then the possibility of a revoke must be checked whether Offender was Declarer or a Defender. The card played in error must be regarded as being in the Offender's hand continuously from the trick of the misplay until noticed and restored. This can result in a revoke for the misplayed card. The applicable rule is:

Law 67 - Trick Either Appropriate in Error or Defective
A trick containing more or fewer than four cards is defective. When one player is found, during play, to have fewer or more cards than all of the other players, the previous tricks should be forthwith examined, face down; if a defective trick is discovered, the player with a correspondingly incorrect number of cards is held responsible.
The defective trick is inspected face up and
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B. after the responsible player has played to a subsequent trick, the ownership of the defective trick cannot be changed and


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if the offender has played more than one card to the defective trick, he withdraws all but one card, leaving the highest card he could legally have played to that trick. A withdrawn card may become a penalty card (Law 50); such a card is deemed to have belonged continuously to the offender’s hand and failure to have played it to an earlier trick may constitute a revoke.