I came across the phrase over-egg the pudding today, and it caught my eye as a terrific Britishism; I don’t know that I know an American idiom that quite gets at the same meaning. If it isn’t clear, to over-egg the pudding is to wreck something in an attempt to improve it, specifically by adding something positive. It isn’t the straw that broke the camel’s back, at least in the way I’ve heard that used, where the straw is usually small but negative. No, this is too much of a good thing combined with trying too hard, only it’s a wonderful and vivid expression.
As a side note, I like that you cannot egg a pudding or under-egg a pudding, only over-egg it. I don’t know if there’s a general rule about verbing ingredients… I think you can verb some, but not all, things that are added to a finished or nearly-finished dish. One can sugar one’s tea but not milk it, for instance, and you can salt your soup or pepper your stew but you cannot ketchup your burger. You can flour your hands, and I believe you can flour the table, but I don’t think you can flour your dough. One cannot over-suet one’s spotted dick, nor over-currant one’s clootie; such a thing would over-egg one’s rhetorical pudding indeed.
But the thing that I really enjoyed about the phrase was something that came up in an in the discussion forum of The Phrase Finder (an enjoyable site, by the way), when somebody using the name Yan Torsen asserts in April 2008 and again in May 2009 that the egg in the phrase does not refer to the foodstuff but to the verb, as in to egg on, from the Anglo-Saxon verb eggian, which the writer claims means to agitate. Thus the phrase would warn against addling the contents of a pudding-skin too roughly, for fear of breaking the intestine in which such a Saxon pudding is cooked, such a comestible being essentially a sausage, without eggs.
This is of course nonsense—the derivation isn’t correct, for one thing, and for another there does not appear to be evidence the phrase was used any time before the existence of pudding-cloths. Also, the verb egg is much more likely from Old Norse eggja, which means incite rather than agitate. Still and all, it led to a respondent using the handle RRC asking how did we get from “over excite the sausage” to the current meaning of “to spoil something by trying too hard to improve it”. Over excite the sausage! The phrase over-excite the sausage (and I have added a hyphen to more clearly parallel the pudding) has brought me more joy than any phrase I have read today, and that’s pretty impressive considering I’m partway through Catherynne M. Valente’s Deathless, a book whose prose may indeed over-excite the sausage somewhat.
I, for one, would like to nominate over-excite the sausage for the American version of over-egg the pudding. This would particularly be useful for describing the irritating addition of an extraneous superhero (or supervillain) in a movie or television series, such that the casting, f’r’ex of Dwayne Johnson in Shazam! would risk over-exciting the sausage.