In working up my new book, I ran into a situation that was unusual and that I have never seen before. You might find this interesting.
Suppose that you have an agreement where after a 2C opening and 2D response Opener's 3C rebid is natural but denies a four-card major. Opener will have 6+ clubs unless he has 5D/4C. In my new approach, that definition exists. But, for those who just play that Opener's 3M rebids show that major (four of them) and diamonds, your 3D bid will have that same limitation.
In the club-suit situation (3C), this creates an interesting handling issue. It seems to me that Responder, if holding a five-card major and four diamonds, should actually bid 3D rather than his major. Consider why. First, if opener has 5C/4D, your diamond call will allow Opener to know that the diamond fit exists, which can be held in reserve until later, such as if Opener makes some other call and only then bids 4D, with the added benefit that Opener could complete his pattern before setting diamonds as trumps. Second, Opener, whether he has a diamond suit or not, will still, if he knows that you would do this with a five-card major, bid his three-card majors up-the-line, both because he wants to find a major fit and perhaps because he wants to complete his pattern. With 5C/4D and 3-1 or 1-3 in the majors, wouldn't Opener bid that major, having already denied four, if he knows that you could have five of them? So, the major fit will be found anyway, but this way Opener will actually declare that possible major contract, which is a good thing. Third, if a major fit is discovered, then Responder will have added to the discourse by describing nine of his cards, which is also nice.
I don't believe I have seen an approach where three-level canape responses with diamonds and a longer major make sense, but this is one example.
By the way, you probably noticed that this same technique does not help when 3D denies a four-card major, which is the common experience of many people. In my methods, I handle this problem with a 3H rebid showing a heart fragment and 4C/5-6D and with a 3NT call showing the same pattern but spades instead (or 4S or 5S with huge hands) because these calls are not really needed for any other meaning, or at least not that much.
I, of course, have suggested follow up agreements. You can't introduce the wacky without answering the "what next?" question.