Has the partner taken inappropriate action opposite the bid?
Problem 4: Fielding

If a player departs from his agreed methods, it may be accidental (scenario A, known as a misbid) or deliberate (scenario D, a psyche). In either case his partner is expected to continue bidding in accordance with the actual agreement, unless and until the psyche or misbid is "exposed". In other words, players must assume that their partners are bidding in accordance with the partnership agreement unless and until the only plausible explanation of the auction is that partner has departed from the agreed system.

Consider case 3, from W's perspective. E has shown Spades and another, and W has decided to sign off in 2. E now bids 3. Why should this not be a game try with extra values, showing that Diamonds are the second suit? If W has a maximum for his 2ibid, or the knowledge that E has Spades and Diamonds means that the hands fit well, then he should consider bidding 4.

If he has better Spades than Diamonds, even with a poor hand, he should return to 3. It would be abnormal to pass 3 opposite the presumed meaning of the 3 rebid, unless W had, say, only 3 (or fewer) Spades and better Diamonds.

If W takes abnormal action, and it turns out that E has departed from the system and has Diamonds alone instead of Spades and Diamonds, then he has fielded the psyche or misbid.