I pass over all the disappointments of plot, acting, tone, style, historicity, etc. My concern here is with the Latin (and the English) in the inscription accompanying the magic hero sword. Here it is:
This is immediately translated by the royal boy: "One edge to defeat, one to defend. In Britannia was I forged to fit the hand of he who is destined to rule."
Actually, the Latin (and the gist of the translation) is not all that bad. I've seen much worse in movies. We've even got gerunds with ad to express purpose, though the quoque is a tad awkward. There are only two fairly obvious errors. In no way does Britannia qualify as a small island within the rule governing the use of the locative case for cities, towns and small islands. The largest island I can find in any grammar book is Cyprus: Hale and Buck, §449, which offers an example from the Bellum Civile, 3, 106,1: Cyprī vīsum. It is deucedly difficult to find any express statement in any of my grammar books (which include, besides H & B, Gildersleeve & Lodge, Allen & Greenough, Lane, Woodcock, Bennett) offering a touchstone for determining which islands are small and which are large, but it is pretty obvious that Britain can't be a small one. Note that the authors of the wikipedia article (as of 5/12/12) on the locative seem to have overlooked Hale and Buck in categorizing Cyprus as too large to take that case.
The second flaw is "incussus" - the past participle of incutio, incutere, incussi, incussus - to strike upon or against; to strike or dash against; to throw, cast, hurl; to strike into,to inspire with, inflict, excite; to shake, cause to tremble (Lewis & Short). The intended word would seem to be "incusus" - the past participle of incudo, incudere, incudi, incusus - to forge with the hammer, to fabricate (Lewis & Short).
The other point worth discussing - not really a flaw, more of a stylistic thing - is the last line: dative (presumably of reference or purpose) of ille, relative pronoun qui in the masculine nominative singular, future active participle regnaturus (in agreement with the relative) of regnare, and an understood est. One might have rather expected to see something like ad usum regis, possibly ad usum regis futuri. That the sword names itself the ensis Caliburnus of Gaius Julius Caesar sets up the irony which a more overt reference to rex would have driven home, since one line of traditional thinking has it that Caesar's monarchical aspirations, or at least the perception among certain patricians that he had monarchical aspirations, got him killed. On the other hand, this level of irony is probably way beyond the film's creators.
As for the English - which is really rather more offensive than the Latin - some quoting (unfortunately somewhat extensively) from the 1927 Fowler, p. 68, will elucidate the issue for those who don't already grasp it:
3. Specimens of case mistakes.I.e., "of he who is destined to rule" should be "of him who is destined to rule." Were it just the boy uttering this "elementary blunder" one could perchance chalk it up to the declining standards of English instruction in American schools. But Colin Firth repeats it. Even the villain repeats it. I think others repeat it. Accordingly it must have been in the script and not a flubbed line by a normally - that is to say inadequately - educated American middle-school aged youth.
A. Three years of dining are a preliminary for he who would defend his fellows. / Should not a Christian community receive with open arms he who comes out into the world with clean hands & a clean heart? / They came to fight in order to pick up the challenge of he who had said 'Our future lies on the water.' / But it is the whimsical perplexity of Americans contemplating the appearance of London that provides he who guides with most amusement.
* * *First in frequency & deadliness comes the personal pronoun in a place requiring the objective case followed by a relative that must be subjective, when there is a temptation to regard he-who or they-who as a single word that surely cannot need to have the question of case settled twice over for it; & hazy notions of something one has heard of in classical grammar called relative attraction perhaps induce a comfortable feeling that one will be safe whether one writes he or him. That is a delusion; neither relative attraction nor inverse attraction (the right term here) is a name to conjure with in modern English grammar, though the textbooks can muster a Shaksperian & Miltonic example or two; in modern grammar they are only polite names for elementary blunders. All the A examples should have him instead of he.
It makes me want to throw stuff at the screen, shouting "of him! him! HIM! you idiot!" And then I right away I feel bad for calling King George VI and Jack Worthing and Mr. Darcy an idiot, and I reprove myself for being such a snoot. I show the movie very seldom, partly just to avoid the ambivalence this particular gaucherie creates for me. But even after I've calmed down, I still can't help wondering how someone who has learned reams of material from Wilde and Austen can keep from gagging when he has to say a line with such a stupid mistake in it.