I've been spending a good amount of time lately looking at the John Rylands University images of their copy of Forme of Cury (I know, what a surprise) and comparing it to other versions. There are lots of differences, but they're usually minor changes in spelling, with the occasional dropped or added word here and there. Sometimes those dropped words can have a huge impact on a recipe.
A perfect example is the recipe for Payne ragoun. The Rylands manuscript has the following:
Tak hony suger cypres & clarifye it to gider & boyle it with esye fyre & kepe it wel from brennyng & whan hit hath y boyled a whyle tak up a drope ther of with thy fynger & do hit in a litul water & loke yf it hong to gider & tak hit fro the fyre & do therto pynes the thryddendel & poudour ginger, & stere it to gyder tyl hit bigyne to thyk and cast it on a wete table, lesche hit & serve hit forth with fryed mete, on flesch day or on fysch dayes.
Compare this to the version transcribed by Samuel Pegge in 1780:
Take hony suger and clarifie it togydre. and boile it with esy fyre, and kepe it wel fro brennyng and whan it hath yboiled a while; take up a drope therof with thy fyngur and do it in a litel water and loke if it hong togydre. and take it fro the fyre and do therto the thriddendele an powdour gyngener and stere it togyder til it bigynne to thik and cast it on a wete table. lesh it and serue it forth with fryed mete on flessh dayes or on fysshe dayes.
Weird spelling and ampersands aside, there's not a huge amount of difference between the two ... except for one word. The Pegge edition leaves out the word "pynes". The omission of this one word turns the recipe from pine-nut brittle into spice candy, and hides any connection to similar recipes for "Pynade" in both Forme of Cury and Two Fifteenth-Century Cookery Books.
Interestingly, the word "pynes" is included in the edition of Forme of Cury that appears in Hieatt & Butler's Curye on Inglish (I haven't had a chance to check which manuscript that transcription was based upon), so a correct version of this recipe has been available for quite some time. Still, I'd hazard a guess that the Pegge edition is the one most often consulted (because it is available online for free).
So what's it all mean in the grand scheme of things? Probably that one should always check multiple sources whenever possible.