I know at least for the English language, there was a long period where words had no agreed upon spelling. But once we enter the current era of formalized spelling, do we have records of any important documents containing overlooked typos and/or spelling errors that were not immediately fixed?
If so, have they had any effects?
It may help to define what you mean by "historically significant," but misspellings have happened. The US Constitution has a couple of misspelled words including "Pensylvania" in the signing section (look in the bottom right corner of the last page). This was a legitimate alternative at the time, but it was spelled "Pennsylvania" earlier in the document: look at the first page, fourth-to-last line of Article I, Section 2.
Additionally, in Article 1, Section 10, second paragraph, it says (emphasis added):
No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it's inspection Laws
You need to zoom in a bit to see it (second line, third word), but it's clearly there. This is a case where alternative spelling can't enter into it.
There was also a thread from a couple of weeks ago about errors in ancient texts with some discussion about exact transcription (including errors) by scribes.