What is the penalty for failing to follow suit?

Question

Assume the mistake is unintentional, what course of action should be taken when this occurs and is discovered?

If the person discovers themselves that they earlier made the mistake, is the penalty different than if an opponent discovers it?

Answer 1

In Duplicate Bridge the penalty for a revoke (failing to follow suit) is very clear.

The official rules handle revokes in Laws 61 to 64. Wikipedia's summary of 61-64 is

A revoke may be corrected (correct card substituted) without trick penalty before any player of the offending side plays to the next trick; otherwise, it becomes established. If a revoke is corrected, the exposed card becomes a penalty card and the opponents may change their played cards as they see fit (however, the revoking side may not take advantage of those seen cards—see Alcatraz coup). When a revoke is established, in general, one of subsequent tricks won by the offending side is transferred to the opponents. If the revoke card has won the trick, that trick is transferred too. (If the offending side did not win any subsequent tricks, no penalty is assessed). Additional tricks can be transferred if the revoke has caused more damage to the opponents than was redressed by those penalties.

If the revoke is not immediately corrected, there is no difference in penalties based on the person who establishes the revoke. It makes no difference whether the mistake was intentional or unintentional.

Answer 2

Failing to follow suit is called a "revoke" or "renege" when that person subsequently plays a card from that suit (assuming that the mistake was not discovered and corrected in time).

The penalty is normally one trick.

But there are two major exceptions:

1) The "reneged" card is placed face up on the table, to be played at the next opportunity (when the suit is led). If it "wins" the trick for the offending side, that trick is, instead, awarded to the injured side for a second penalty trick. (If the offender's partner wins the trick, they get to keep it.)

2) If the offending side loses the reneged trick, and all tricks subsequent to the discovery of the renege, the penalty is zero. (The offense didn't cause any damage.)