I've just run into a problem involving a question with an accepted answer that is demonstrably (albeit circumstantially demonstrably) untrue: Who first said "We can predict everything, except the future"?. The complication in this instance is that the question has been closed, thus effectively locking out competing answers that might be more accurate and therefore more useful to site visitors.
As long as the question and answer persist in their current states—the question closed as off topic, with a single answer given and accepted—EL&U is playing host to a kind of hermetically sealed error. Since no further answers are permitted, the most another site user can do to combat the misimpression that the accepted answer leaves is to respond to it in comments—unstable and (often) short-lived though they be. I have done so in this case, but the result isn't very satisfactory.
To his credit, the answerer clearly identifies where the longer quotation that he offers as the source of the poster's question came from: "I found this on a mailing list and someone had posted it." Nevertheless, he incautiously argues that "it appears to have been written by John Galsworthy" because "wikiquote seems to suggests his work Swan Song (1928) Pt. II, Ch. 6 for the last line."
In reality the block quote that the poster attributes to Galsworthy is a gallimaufry of sayings about the future from disparate sources (most of them unknown), concluding with a sentence from Swan Song. One of the earlier sayings in the compendium is the one that the OP was interested in tracking to its origin—so that wording receives the answerer's "done by Galsworthy" sticker too.
The simplest solution to the problem would be for moderators to delete the question and answer (if it's possible to delete questions that are closed but have accepted answers). But I'm not persuaded that this question (which was closed back in July 2012) is off topic by current site standards.
The OP asks for the source of "we can predict everything but the future" as a quotation (and uses the 'quote' tag as his tie-in to approved English Language & Usage topics). EL&U regularly fields questions that ask about the origin of sayings or expressions or proverbs or (in some cases) quotations, without rejecting them out of hand. In fact, the quote tag has the following thumbnail description: "Questions related to (semi-)famous quotations." The saying that the poster asks about seems to me to fit squarely into the area of inquiry that the 'quote' tag expressly covers.
As I write this, only 545 people have visited the page over the course of 4 years and 8 months, so civilization is unlikely to mourn its loss if the question does get deleted. Nevertheless, since our site put the misinformation out there in the first place, I think we have some responsibility not merely to erase the infamy but to correct the record.
Moreover, if one person casting about for the source of "we can predict everything but the future" came to the conclusion that it was probably written by Galsworthy, others might—by the same or similar missteps—reach the same erroneous conclusion. May we not therefore consider it an act on behalf of truth, justice, and the [adjectival form of your beloved country goes here] way to cast at least a feeble ray of light into that obscure corner where misapprehension has so long dwelt?
I recommend reopening the question so that interested people can submit better answers to it. Failing that, I recommend deleting the question. It doesn't do our site's credibility any good to entomb fatally flawed answers so that they lie rigid and authoritative in the vaults of closed questions, ready to waylay unwary solitary wanderers who may happen upon them.