Project Gutenberg Nodded Off

"Even Homer nods off."
-Dryden butchering Horace ("indignor quandoque bonus dormitat Homerus". Horace, Ars Poetica II.259)

Let lectores inscribe this lesson in the forefront of their minds.
There are bound to be mistakes in even the most hallowed of masterpieces and often those errors become endearing bits of trivia. Some of these mistakes, however, can be chalked to manuscript "oopsies" which can cause both delight and consternation.
That said, given how grand an achievement Sir William Stewart Rose's translation of the Italian epic Orlando Furioso is, the work is bound to possess several cute little authorial idiosyncrasies of its own, as well as editor errors. For example, the nerds at my school's Epic Poetry Recital Club cried foul when they were reading Rose and found that stanza VII of canto 13 is short a line.
While whining indignantly about being told that the poem is written in ottava rime and ottava means eight, the students counted out the seven lines:

“When him I after in the field espied,
Performing wondrous feats of chivalry,
I was surprised by Love, ere I descried
That freedom in my Love, so rash a guide,
I lay this unction to my phantasy,
That no unseemly place my heart possest,
Fixed on the worthiest in the world and best.
                                               -Rose 13.VII

Sure enough; the boys were right. The text as written on Sacred Texts has only seven lines. We then checked Project Gutenberg's edition and found, indeed, only seven lines. The University of Adelaide follows suit.

We then went to the original Italian, which reads as such:

"Il qual poi che far pruove in campo vidi
miracolose di cavalleria,
fui presa del suo amore; e non m'avidi,
ch'io mi conobbi più non esser mia.
E pur, ben che 'l suo amor così mi guidi,
mi giova sempre avere in fantasia
ch'io non misi il mio core in luogo immondo,
ma nel più degno e bel ch'oggi sia al mondo."
                                               -Ariosto 13.VII

There's that line missing from Rose's translation: "ch'io mi conobbi più non esser mia". It seems Ariosto's ottava rime is just fine; did you punt this, Rose?

It turns out, no: the problem, it seems, lies with Project Gutenberg, which incorrectly copied their edition, and so all successive editions which follow their version have retained the error. 
Archive.org (pg. 214) has the correct text:

“When him I after in the field espied,
Performing wondrous feats of chivalry,
I was surprised by Love, ere I descried
That freedom was forever lost to me
Yet, following in my Love, so rash a guide,
I lay this unction to my phantasy,
That no unseemly place my heart possest,
Fixed on the worthiest in the world and best.
                                                  -Rose 13.VII

Here we can observe the viral and destructive path the misprint wreaks, tainting each successive copy, "corruption breeding corruption" (to quote Tom Stoppard). Let us always be watchful and constantly check our primary sources. 
This is why we read works in their original languages.